Admiring someone is part of the life cycle. Learning how to deal with it is a valuable lesson. Being part of the conversation is always key, because you know that your child will come to talk to you in later years.
I hate to appear controlling, but sometimes children just don't know what is really good for them. If you let your child to deal with this on their own, chances are they might get hurt, or it will affect their grades. Anyway, I believe parents should maintain a good relationship with their children, and where possible, give them necessary advices along the way. Be a friend as well as a parent, and I'm sure that the child will open up to you instead of getting rebellious.
Do children even know what crushes are? I would just smile at my children if they came home and told me that they have a crush on their teacher. I would try to give it a different name and let my children know that admiring a teacher does not equate to love/crush/infatuation.
My daughter tells me she loves a boy at kindy and wants to marry him one day. It's so innocent at this age that I'm not remotely concerned. I haven't even thought about when my kids would start dating yet. It seems a long way off though no doubt it will come quickly. I would hope that isn't an issue until they're teenagers. If my ten year old had a crush I suppose I'd encourage them to tell me about it and listen patiently. Beyond that I don't know.