Stupidity: Principal Fremont did not even consider the possibility that what Mr. Butler was saying about Lucas could be true. She basically laughed it off ("dismissed" it as absurd). On different occasions, Mr. Butler tried to tell her that he didn't think Lucas was who he pretended or appeared to be and that Lucas was blackmailing him for an 'A' in his English class. Because all of Lucas' grades from his previous school were 'A's, the Principal took for granted that they were legitimately earned despite Mr. Butler's concerns. A school Principal should be willing to at least look into Mr. Butler's accusations against Lucas, not merely blow them off as false. [The fact that Mr. Butler could not be more convincing is also problematic.].
Stupidity: Why didn't they take out the crushed hand guy while the creature was fumbling inside the glove? Plenty of time to do it. Why did no one use a knife to stab the creature?