Character mistake: Brandon begins the scene giving his speech while he is sitting/leaning on the desk. The students all move into the picture from both sides and gather around Brandon then they all say, "There's no hope with dope". After the conversation with Mr. Belding, they watch the commercial they just shot (beginning with each student giving his/her opinion about drugs). Then Zach says his speech while sitting/leaning on the desk. He stands up, the rest of the students and Brandon move in and gather around Zach, then Brandon gives his anti-drug speech.
Continuity mistake: At the end when they finish the commercial, nothing at all is the same on the playback as what they showed them do. None was the same. The wording and also the places and stances everyone was doing. The entire thing was different in the playback.