It's just a personal opinion that I think you should of let her get more weight on her before riding, regardless of size.
well that is yours, the teacher thought she was ready to start riding, and for a LONG time it was walking in the round pen and walking up and down the hill, then slowly we started to get her to trot, and now, it is tis time for her to go faster then a fast trot, trust me, she can do it, she did it on me when i just wanted her to trot, she went loping LOL