Listen, do you have a record of the community service you did for the City?
Can you go and have them write you a letter explaining what you did and that you fulfilled your obligations?
If so, DO THAT before you attend this little school meeting because they are not going to believe your words, only official documentation. Bring like 3 copies of everything. Cover your ass.
Sprite
PS, I am getting the feeling that they may perhaps be trying to just fuck w/ you and scare you a little bit. Power trip thing that people do. I will not be one bit surprised if you show up w/ your proof of stuff and they let you go with some additional fine just to sort of slap your ass on the way out (in a bad way)