When The Mindbenders play the graduation dance, they are very
obviously performing a lip-sync to studio recordings. A piano is audible, though no one visible is playing one. The guitar heard is a 12-string, though Eric Stewart is seen playing a 6-string Gibson Les Paul. Eric Stewart's lead vocals are very obviously double-tracked, a common technique for hiding performances with pitch issues.
During the dance between Thackeray and Pamela, when The Mindbenders abruptly switch from a slow song (opening bars of "Off And Running") to a fast one ("It's Getting Harder All The Time") we hear the students cheering, but the shot at that moment is of the whole class, and no one is cheering.
When another student throws a can at Pamela, she is facing Thackery and defends against the object with her right hand. When Thackery catches the can a split second later, Pam now has her back to Thackery, defends with her left hand, and has to turn completely around to respond to him.
After Thackery punches Denham in the stomach during the boxing match, he tells Denham to put his head down. Denham then bends over at the waist. When the shot changes to a more forward view Denham is more upright & Thackery again tells him to put his head down. There was not enough time for Deham to have straightened up.