Are you tense before your class? There is potential that it has nothing to do with your horse, but more or less with yourself and nerves. Even the best of us get nervous before entering a class, and that may transfer to your horse, making him worked up and worried, and as he gets more nervous so do you, and it turns into a nasty cycle.
Try to work on breathing and relaxing. Chances are you're relaxed while warming up, and that's why he's behaving fairly well for you during that time.
EDIT: Did the trainers bring him to shows? I didn't catch that part. If they did and he acted the same way, I'd also be curious if he has ever been shown successfully.
Last edited by Moocabear; 08-19-2013 at 08:09 PM.