"You're not gonna look bad," he assures, lingering under the hoop as if ready to defend, but just watching her instead. He figures that deep down, no one can be terrible at basketball because basketball is awesome and pretty much everyone in his school- including his girlfriend- are pretty good at it. East High has messed up his judgment on all of life.
Just like he promised, he's not even laughing at her attempts, he's just waiting like a good boy until she takes the shot, watching it's rebound and bouncing after the ball to retrieve it.
"You don't suck! That was just some, uh, not so brilliant aiming. You just gotta learn how to get the ball in the hoop, right?" To be fair, he deserves awards for not laughing at that squeal she did.