In response to this question on RTR, I'll also include some of my previous comments below on high school in general that I made when talking about possible ways of making CTC high school credit worthy. When I'm typing, I'm not purely addressing you as I'm thinking about RTR overall from a high school perspective, knowing that families in all different situations will read this.
Making CTC or RTR credit worthy will depend upon what your state requirements are for high school (and also what type of higher education plans you have for your child as to whether that child is college bound, etc).
For a distinguished track high school education, most states are moving toward a 4 x 4 requirement for serious college-bound students who wish to compete for scholarships. This includes requiring 4 years of science (with 3 of the years being a lab science and typically including Biology and Chemistry at a minimum), 4 years of literature study/grammar/composition combo. (with American lit. being a requirement for sure and some leeway in the other literature areas), 4 years of history (typically covering geography a year or at least a semester, world history a year, American history a year, and Economics or Finance/Government each for a semester - although you can do various combinations to reach these requirements), and 3-4 years of math (with a bare minimum of algebra and geometry). There are other additional areas that are typically required as well like speech, fine arts, physcical education, computer, 2 consecutive years of a foreign language, etc. however the 4 x 4 requirements will take the bulk of the time.
If your student is not college-bound or is headed toward community college, you would have more leeway in the requirements and would instead focus on the basic requirements of the state and on fine-tuning a course of study that will guide your child in training for his/her future goals.
As far as RTR goes, in order to count it for high school credit, you'd likely need to add a bit to the history readings to make it credit worthy. There is quite a bit of reading within the history portion of RTR so not a large amount of additional reading or materials would be required. You could add something like Drive Through History DVD: East Meets West for the first section of the study, and add something like The Story of Europe: From the Fall of the Roman Empire to the Reformation by H. E. Marshall (Yesterday's Classics) along with Courage and Conviction: Volume 3 by the Withrows to accomplish this.
Another way to raise the level of this study to being credit worthy and still match this time period well would be to do Pilgrim's Progress. You could do this using the Answers in Genesis Study (which we plan to schedule in the last guide of the history cycle before high school), or you could simply have your child do it following along with book and audio instead. The AIG version has study questions and an audio as well.
Depending on how much time you spend on this, it could be a large part of a literature credit or a Bible credit, or be the only thing you add to raise the history level.
As far as geography goes, there is quite a large amount of mapping within RTR so there would be no need to add anything to that area. The geography within RTR alone would likely be 1/4 credit of your total geography needed for high school, as you'd be doing some geography each year to add together for your geography credit by the time your child is a senior. In that respect, the geography would likely be fine from RTR. The geography portion in our upcoming guides will be strong too, so you'd likely be alright if you kept on this same path of upping each level of HOD for your high school child.
You'd also want to make sure your child is reading higher level literature, doing this either through choosing higher level books that match the time period (or any books you choose) to add to DITHR or by utilizing a different literature program instead. DITHR is worthy of high school credit in lit. for grade 9, depending on which books you select for use along with it. Some ideas of a few classic high school titles that would match the time period well may include A Connecticut Yankee in King Arthur's Court by Mark Twain, A Merchant of Venice by Shakespeare, Men of Iron by Howard Pyle, The Vicar of Wakefield by Oliver Goldsmith, The Black Arrow by Robert Louis Stevenson, Kidnapped and Treasure Island by Robert Louis Stevenson (although these are sometimes used in middle school), The Prince and the Pauper by Twain, and a tale of Arthur such as Lancelot: The Adventures of King Arthur's Most Celebrated Knight by Chaundler (for its less magical themed telling). Jane Eyre by Bronte and Silas Marner by George Elliot are excellent and are about 200 years ahead of the end of this time period. Both Don Quixote and The Three Musketeers fit here as well but are very lengthy.
Since we've used DITHR for years for my oldest son, we are still deciding whether to move into something else in high school for lit. study. We have looked long and hard for one that we like for our own son and have been having a tough time finding one that meets all of our goals. We are considering Smarr's Intro. to Lit. course at this point, however we prefer something else for grade 9 comp., rather than Smarr's writing lessons (at the 9th grade level anyway). Updated to add that we did do Smarr's Intro. to Lit. with some substitutions and did do Smarr's comp. and liked it quite well.
Another option you could consider for literature study would be to do Peter Leithert's Brightest Heaven of Invention (which uses entire Shakespeare plays along with a Christian study of them). This is definitely high school credit worthy and would work well for this time period for the literature study. As a stand alone literature credit or combined with a read through the book English Literature for Boys and Girls by H.E. Marshall (85 chapters) it would give a full year or more of high school literature instruction. The English Literature book could also be spread out over several years instead, jumping off at the end of various chapters to read the full length suggested works as desired for credit. We like this plan ourselves.
IEW's Medieval History-Based Writing Lessons as scheduled in RTR with Extensions is enough for the high school composition part of the English credit. When combined with literature study and grammar instruction it will equal a full english credit.
For grammar, we would recommend continuing with Rod and Staff, doing English 7 during grades 9-10 (half each year) and doing English 8 in grade 11-12 (half each year). These two levels are definitely high school worthy in their Table of Contents and in their coverage. You could use any grammar option with which you feel comfortable, although I'd lean toward doing some grammar every year as many sections of the SAT/ACT are a reflection of what is covered under the "English" umbrella.
You would definitely need to add or use something else for science as earth science is typically not enough in high school. Much of your decision-making in science will depend on whether your child is continuing on to higher education after high school and if so in what field. Some families may consider using Signs and Seasons along with the provided labs in the Appendix of Signs and Seasons for credit. This would work if your child is not likely going on to higher education after high school. For ninth grade, we are leaning toward either PAC-works Integrated Physics and Chemistry or Biology and Anatomy and Physiology. The Biology would need labs on Day 5 each week, possibly with Castle Heights Biology labs. Or, you could use any other program you feel suits your needs (i.e. Apologia, BJU, Abeka, etc.).
For Bible credit, you could add something like Balancing the Sword to the Bible Quiet Time readings within RTR. This would add time to that component to make it credit worthy and raise the level of the material, while giving some good thought-provoking questions to go with the Bible reading. We are planning to use this with our own son for high school Bible study at some point. Or, you could simply add some additional Bible reading and journaling (along with the Philippians study and Boy/Girl option devotional scheduled in RTR) to earn the Bible credit this year since RTR has some of this already taken care of for you.
For a fine arts credit, (in addition to the once weekly Looking at Pictures study in RTR) in the area of art appreciation, you could add Short Lessons in Art History by Phyllis Clausen Barker along with Exercises and Activities for Short Lessons in Art History. It does include some nudity, and we have not previewed it thoroughly yet but it is a possibility. Or, you could add Calvert's A Child's History of Art course (which includes Painting, Sculpture, and Architecture). We have used this one with our oldest son doing one section each year, or it could be done all in one year. Either way it would be credit worthy. We enjoyed this one quite a bit.
Just a few thoughts on RTR.
One last thought I had was that for those of you possibly thinking of using HOD's younger 4 year sequence of guides for high school instead, it would likely work (with some beefing up as suggested) to give 1 full credit of world history for CTC and RTR together. Then, you could likely give one full credit of American history for the last two guides in the history cycle together, also awarding 1 full geography credit upon completing all 4 guides (with 1/4 credit awarded each year). That would only leave you to add a semester of government and a semester of personal finance or economics to complete your "history" portion of the 4 x 4 plan. Another option would be to just give a history credit each year that you use HOD in high school (as you would definitely have the time needed to earn one credit each year) and just make sure that you cover all needed areas (i.e American, World, Geography, Government/Economics or Finance) within the 4 credits that you award throughout high school for the history portion.