I would generally advise against taking your ball python out in the general public. A large part of the population has an irrational fear of snakes, and taking them in public will almost certainly trigger some intense reactions. With all the legislation going on around keeping reptiles in general, it's just not a good idea to give them any more ammo.
A show and teach day at school is entirely different. I would suggest talking to the teacher about it first to make sure they would be ok with it. You should develop guidelines for how you will be presenting. Will you allow the kids to come up and touch the snake if they want to? Make sure you think of things like hand sanitizer before and after if they're going to touch. I don't think you have so much to worry about from the snake as you do with the teachers/administration/parents...