I am not speaking for Seantrel, I have no idea how smart he is or isn't. However, a lot of parents send their kids to Cretin to keep them out of trouble and to maximize their scholastic ability. You rarely see a Cretin kid get into too much trouble (I know the Floyd and Shady thing), but out of the countless Cretin kids, they generally stay out of trouble.
The private school atmosphere I think helps get kids who probably wouldn't be able to graduate HS to get to a point where they can graduate HS.
Furthermore, if the kid is having a hard time getting the minimum to play college football, that really isn't that rare. The national average on the ACT is 20, and I would have a hard time imagining that most kids who aren't graduating HS are even taking the ACT. There is obviously quite a few kids who graduate from HS who wouldn't be able to get a good enough score to pass the NCAA clearinghouse. Granted, Cretin should have a better than average on their ACT (i'm sure they do), but that still doesn't mean that there aren't quite a few kids who graduate and wouldn't get a good enough score.
Lastly, what would Cretin's incentive be to cheat? Yeah, they get a bit more exposure than they normally would, but they were already getting a ton of exposure (locally and within college football recruiting circles). There probably won't be a kid getting as much exposure, but Cretin has had and will continue to have tons of kids like Ryan Harris, Michael Floyd, Joe Schafer, Rashon Powers-Neal, Phil Archer, Broderick Binns....why would they put their entire credibility in jeopardy, for 1 kid? They wouldn't...their is flat out zero incentive.