You know, it may well have been that the owner of the school was simply pre-occupied and didn't see or recognize you. Trust me, that does happen when a person is trying to keep a business together.
Now, about Duncan. I would call the school and explain how you get really attached to him before your parents decided that you should stop going there, (better not to mention why if you can help it) and ask if it would be OK to come and visit him. You do know that horses age about twice as fast as humans? It may not be terribly long before he gets to be too old for working in a school. If the school owners seem to treat you decently, talk to your parents and see what they think of the idea that you might buy Duncan when he retires from the school. If they seem to be agreeable, then talk to his owner about it.