Riding help please: the horse I ride continually tries to canter out the gate.

I ride English and the lesson horse I ride darts out the gate whenever I am cantering on him. I have tried to pull as hard as I physically can and it only works once in a while. He is a great horse and it is my fault because the first time it happened I was careless and didn't think that he would try it, but now that it happened once he thinks he can do it every time. Please keep in mind that I can not work with him outside my lessons because I do not ride him. Thanks, I really appreciate any advice!!