Barry said that if Bart wanted to live with he and Iris, he had to go to school like a normal teenager-- which wasn't unheard of, Max had set the same rules, and Bart was a little more willing to obey them for the sake of his grandfather. So when Barry had put him in a variety of sports in an effort to keep him active and to help him integrate with peers his own age, Bart had managed to do nothing other than put his head down and pull his weight-- despite the fact that he'd much rather be at home playing video games. What was worse, he got put in track and field, where he was supposed to run but not win, even though he could, easily.
Most of the time he could fake normalcy long enough to see his Grandpa Barry be proud of him from the stands and that was worth a lot -- but other days it was far too much to try to squeeze himself into the role of the athletic, completely and totally normal high school freshman.
Besides, he could move at super speed and no one would see him leave. And it would just be for a little while anyway.
One second he was standing on the field after one of his races (ha! Could you even call them that? Booo-ring!) and the next he was outside of the gym, still in his track shorts and jersey and... was surprised to find someone else there.
"Uhhhhhhhh......." Hopefully she hadn't noticed that he'd just... come up out of nowhere. "Hi."