I agree with koomy56.

I think a lot of it definitely depends on your instructor. I almost always had group lessons growing up and we got to about 2'6" as young hunter/jumpers.

Your instructor might also be thinking of you and the other riders as a whole. If most of the riders in your group aren't ready to jump any higher you might be forced to stay at the lower heights. In most of my lessons my instructor had to put the jumps up just for me and my mare or make an entirely different jump or set of jumps and it got a bit annoying.

Talk with your instructor and good luck!