Well, everyone, you do know this is a lesson horse, right? I don't know how the stable is run, but around here, most of the horses are used 6 or 7 days a week, all throughout the day, usually in ill-fitting saddles, exceptionally tight girths, and usually with improperly fitting bridles. Couple that with riders who are learning and often haul on the horse, no wonder you've got a sour horse.
Anywho, before you discipline the horse, make sure you're not the problem. Do not stand up in the stirrups or even lean forward when you ask to trot. Try clicking at the horse when asking to trot. Keep your hands down. Don't 'squeeze'. Instead tap. If that doesn't work, give a kick with ONE leg and then try tapping again.