Bi-bi isn't just used in Deaf ed, it's proved very successful in all bilingual education, including hearing. Whether the child uses spoken Spanish or ASL, promoting fluency in a child's L1 from an early age (before age 7) then teaching English as a second language (L2) while increasing fluency of their L1 in a more and more advanced level usually by teaching all other subjects in the L1 including more advanced studies of the L1 itself. In other words, children from Spanish speaking families who have learned Spanish as the first language, will have more success if their Spanish language skills are expanded on, by teaching Math, Social Studies, etc. in Spanish, they learn higher more advanced Spanish by studying literature, grammar and writing in Spanish, and English is taught as a second language. The higher their Spanish skills become, the higher their English skills will be too. For Deaf children, it's important they've been exposed to an accessible complete natural language by age 7 aka ASL. If they miss L1 exposure by age 7 they'll gradually lose thee ability to master any language at all. On the other side, if they're taught at a continuously more advanced level and understand of ASL, then when they learn written English as their second language they'll have an equivalent level of mastery. It's all based on Chomsky's theories of Language Acquisition.
Here's a great study from the Journal of Deaf Education that not only includes it's own studies that support higher ASL fluency results in higher literacy levels but references numerous different studies that support the different components that make up the basis of bi-bi education.
https://academic.oup.com/jdsde/article/21/2/156/2404366 When I started college for Deaf ed in 2006 there wasn't a text book, paper or class that promoted SEE based education at the expense of ASL based education as the overall gold star of Deaf ed programs.
As far as the scores, I wouldn't depend on TCAP scores (CO Standardized Testing) They're very specific and want a certain answer for the reading tests. Schools with teachers who have been in CO for awhile know how teach to the test. A lot of the RMDS teachers are from out of state. I once failed my grade 10 CSAPS (what TCAPS were called when I was in school) because I provided answers that were outside the box. I got a 5 on both my AP Lit and Lang tests, a 36 on my reading ACTs (score out of 36) and a 770 on my verbal SATs (back when scores were out of 800). TCAPs are very controversial in CO especially among educators here.
Also I don't know where you got that 10% score because while your numbers are accurate on every place I've looked for CSDB results, everywhere I've found shows much higher proficiency on state testing for RMDS. If you look at other statistics, it shows vastly superior achievement at RMDS vs. CSDB. For instance 60% of students at RMDS have a 4 year high school graduation rate vs. 19% for CSDB (the number you left out). More telling is that CSDB's graduation rate is declining annually. 100% of graduating seniors at RMDS attend a 4 year college vs. only 11% at CSDB. Parent and student reviews also rank RMDS higher than CSDB. Even if you try to look at the raw TCAP/CSAP combined data from the CDE reports, there's one huge thing that you're not accounting for. Those test scores include the half of CSDB students who are blind/visually impaired. If I were a betting woman the VI/B school at CSDB has much higher reading/writing results that are pulling up the overall average. That probably says more about RMDS than anything if a student body that is 100% Deaf/HoH can be in the range of a student body that's made up of 50% VI students. And CSDB isn't a SEE based school. Their Child Find early ed programs are now all prioritizing ASL fluency first and while they're officially TC, ASL based fluency is promoted.
All that said, numbers often don't show the full story. If you looked at my test scores and success in university (I went on a scholarship to Brown University, I have three bachelors, a BA and a two BSs from CU Boulder, I started law school on academic scholarship at DU and I'm applying to masters programs) you'd say look at this HoH girl, she was taught using SEE and look how well she did, it's proof of it's success. It's anecdotal but I believe I, as well as a couple of my Deaf/HoH friends from our high school's mainstreamed program, all did very well in school. But our parents were all big believers in promoting reading outside school, had high academic standards, etc. I've always loved reading and started reading on my own at a pretty early age, without much or really any instruction. We probably would have done decently well no matter how we were taught. But now as an adult, I feel stuck between two worlds and I don't belong properly in either right now. What I've seen at RMDS is while integrating students into mostly hearing families, there's a huge promotion of confidence about being Deaf, which is aided not just by instruction but because almost all the teachers and staff are Deaf/HoH. Being hearing, it's something that is important to try to remember Deaf identity is something that is unique for each Deaf/HoH that they should have the right to decide on and be provided the tools to allow them to succeed with whatever they choose.