Let's put this baby to bed, shall we? No pun intended...
MC says he's 2 years older that Sara, but he's in high school, which means Sara would be pretty young. Certainly under 18.
UNLESS!!!
MC was held back in school! And honestly, with the issues this cat has, would it shock any of you to find out that he had some serious issues as a child?
So he's a super-duper-senior in high school, Sara is a regular senior. They're in different classes because she's a good student and he's the kind of student that blackmails his teachers into anal sex.
Done and done.