I have a mixed feeling about retconning Eighth to be the reason Sima Yi didn't get into Water Mirror's school.
You know now that I think about it, Sima Yi and Eighth is doing the same thing which is getting into various schools, and studying various texts only the difference is Sima Yi have no ambition prior to his rebellions. From passing lines we know that Yi attend the same school as that kid from Tian family, he also make comments about how he learn religious trick from Yu Ji although this chapter imply that Yi never got to study Yu Ji medicinal skills. More curiously there are several instances that imply Yi actually attended Watermirror schools but not a part of Eight Genius.
Proof: Sima Yi call Yang Qing as his school senior when they met in Guandu, Yang is a close friend of Yuan Fang during their times at the school.
Yi knows Kan Ze and call him master as well, during the prelude to Red Cliff it is mentioned that Zhang Zhao is a master of finance for house watermirror while Kan Ze could predict weather more accurate than anyone (also another instance of Sima Yi studying under others)
Furthermore of a proof that Sima Yi and Eighth action mirror each other is the fact that Sima Yi say he read most of the books on Watermirror library because they were interesting during early chapters. So we do know that Yi have an interest in studying a lot of things but he isn't as motivated because he have no goals. What I find interesting though is Zhao Sheng immediately recognise Sima Yi as a threat and drug Yi immediately making him few of the people that recognised Yi brilliance since childhood. The other who recognised Yi brilliance early are Zhang Fei and Watermirror (ironically)