My daughter isn't school-age yet, but I don't think I'd be worried if a teacher were her friend on Facebook. However, we don't plan to even allow Facebook (or whatever its equivalent will be in 13 years) until our children are at least in high school (or whenever it seems they have the maturity and good judgment to use it safely). We'll be concerned over any kind of inappropriate behavior by any of her teachers, but friending her on Facebook isn't inappropriate in and of itself. Different parents will feel differently, I'm sure.
As for denying friend requests, I guess it's not exactly true that you're not "allowed" to friend current students, but it would violate a rule you have put in place for yourself. If you're concerned about bending the truth that way, I think you'd be fine simply saying, "I'm sorry, but for many reasons I don't friend current students of mine. How about when the term is over?" Or something. There's nothing wrong with denying any friend request that you don't wish to accept.