Bucking when asked to canter?

Ok so my 18 year old arab, has a new habit that I've never seen before... we started using him as a lesson horse because, I don't have the time to work him as much as he needs worked (when trying to keep my ottb from killing me, diff story but to those who know what im talking about he's actually doing better =) yay!) and the owner of the barn needed a back up lesson horse... he does two lessons a WEEK and they are only an hour long. All he has to do is walk trot and canter both ways and walk over a pole for this 13 year old girl. Well about two weeks ago the other lady who is using him for lessons was lunging him and he bucked when she asked him to canter.. we didnt think anything of it, two days later he bucked when I asked him to canter, again didnt think anything we just thought he was feeling good and wanting to release energy.

The 13 year old had a lesson last sunday and he didnt buck when she lunged him, in fact he hadnt bucked at all except those two times, but I guess he bucked when she was on him and asked to canter, she got a kick out of it though.. lol she got to learn what to do when a horse does that...

my mom says we should stop the lessons but I don't think we should.. what could be making him buck? And what should we do to correct it?

Inexperienced riders bumping around on his back could be causing him pain. Just a thought

My mare began bucking everytime she was asked to canter, as soon as you sit for the canter she'd throw one big buck in, and then canter on, we got the chiropracter out and she had a sore back.. maybe that's whats wrong wiith him? Apparently they can get sore anytime, even rolling in the field the wrong way they can get a sore spot.. just an idea (:

My guy was also acting funny at the canter...had the chiro out and he was 100% better. Not that it will be that every time, but might be worth a check

I would have to say that he is sore. Could just be anything he did as mentioned above, or that the rider was bouncing on him which was also mentioned above. I would get a chriopractor out...I can't hurt. And if he's still bucking get a vet out.