How to keep a horse from over-jumping?[also, cantering question]?

I always used to circle my horse if she did that, not a sharp circle, but for example round the menage / field, don't know where ur riding. The instructor is obviously not doing a very good job if your horse is bolting she/he should not be letting this happen and should discourage it from doing that because in the long run it or the rider will get hurt. Try half halts too, ie rather than tugging on the reins, try pulling your 4th and 3rd fingers back slightly this gives more effect. Hope this helps