Schooling show- play it safe with Walk/Trot classes or try a canter class?

I think that if your willing to experiment a bit, go for it! If you can, the day before the show go to the barn where the show is going to be and ride him in the ring/arena that they have there and ride with some other people. The only way you can really find out how he will behave is to play with it a little bit! Also, if he prefers trail rides so much he hopefully will be fine adapting to a new environment on the day of the show. I think its worth a shot, and even if it doesn't go the way that you plan its a learning experience and then you will have a better idea of what to expect from your horse next time, or what you might want to do differently. Good luck in the show!