In To Sir, With Love, who looked at Mr. Bell with anger in PT class?

Mr. Bell is the PT teacher (or gym teacher) at the school, and he was abusive and derisive towards the students—Buckley in particular. As he grows more and more abusive towards the boy, the students begin to get concerned.
When Mr. Bell assigns Buckley a task that is far too difficult to him to perform and begins to scold him, the boy Potter reacts violently, looking at Mr. Bell with anger. He picks up a club and prepares to attack the teacher—which surely would have severely injured the man and gotten Potter expelled—but the other children run to find Mr. Braithwaite, who is able to help calm Potter down.
Braithwaite's intervention is the only reason the event didn't escalate further, but it permanently damages some peoples' relationships with Mr. Bell.