Strickland holds his hands over his ears then puts them down after Marty plays "Johnny B Goode," then in the wide shot of the crowd Strickland has his hands over his ears again.
Just maybe it's because :
1. Something happened and he thought "hey not so bad anymore - then thought damn when Marty started to play again LOUDER"
or
2. Someone was talking to him when the camera was away like a student or faculty member.
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Dan Slep writes:
3. This slipup is right STOP ASSUMING!
This is a clear occurrence of events shot in filming not being presented in chronological order. Well done.