Per Wikipedia Oher had a 0.76 GPA going into his Senior Year. Also, the only reason he got into that private school was this: "the school's football coach, Hugh Freeze, submitted Oher's school application to the headmaster, who agreed to accept him if Oher could complete a home study program first. He did not finish the program, but was admitted when the headmaster realized that his requirement had removed Oher from the public education system." Sounds like Hugh Freeze being Hugh Freeze.
I'm not saying the kid was naturally dumb (he did pretty well in college), but it's not like he was even below average up until his senior year of high school. He was failing. And the Tuohys hired a private tutor to work with him for 20 hours per week.
If it were not for them, he would not have been able to go D1 due to grades in my opinion.