Gah. Have you talked to the preschool staff about this? I think if they're worth their salt they'll be able to deal with this, e.g. by talking explicitly to the children about how different people are ready to do different things at different ages and that's OK! It's more than just "don't be rude to your friend" at stake, it's acceptance of individual difference.
This was never an issue at C's preschool - I remember other kids commenting on C being able to read, but never in a nasty way. When he started school, one of the things his teacher did in the early days was to ask all the children what they thought would be good about learning to read, and they all (with help!) completed a sentence about it. Colin's was "because when you want a story but nobody wants to read it to you, you can read it yourself"!
