Naturally, after every "Happy birthday Ms. S!" came "So how old are you?".
My standard response was, "Older than I was yesterday." That only worked sometimes.
5th grade girl: So Ms. Sobers, you're 19 right?
Me: No...
5th grade girl: So then how old are you?
Me: Older.
5th grade: 33? 27? 45? 90? 103?
Me: Yep. 103 you guessed it.
1st grader (passing by): Wait...103??? Aren't you going to be dead soon?
Me: No, because I'm a vampire. I don't age.
I did fess up my true age to one class. Only because they started that song where they say "Are ya one? Are ya two?...." and I couldn't let them spend all that time singing, because I did need to teach them!