I am a Single Mother by Choice, having used an anonymous sperm donor to conceive my child. He's now in second grade, and I have found this tip to  be handy in our situation.  Invariably, a classmake will ask my son where his dad is or why he doesn't have a dad.  My son is very comfortable with these questions, and generally just tells the other kid that he doesn't have one.  The other kid often tells my son that that is impossible, and everyone has a dad somewhere. 

I have found it very useful to talk to the teacher beforehand about this and make sure that he/she validates my son's side of this conversation.  I don't expect the teacher to explain anything specific about my son's situation, but I want him/her to say something like, "No, Peter is right. He really doesn't have a dad. Most of the time kids have dads somewhere, but there are cases where that's not true." 

This validation of my son's situation is crucial!