In the scene where Marty and Biff are in the cafeteria, Biff pushes Marty and Marty pushes back and they stop when they see Mr. Strickland but they still have ahold of each other's shirt. When the camera is pointing at Biff's face and Marty's back, Marty is no longer grabbing Biff's shirt. In fact, it doesn't even look like his shirt was ever touched.