Show & tell precautions

There really isn't much to add to the above though I did this for many years with my corns.
The ones that I use for educational purposes are handled constantly, will allow me to 'squish' their head, and squeeze a bit on the body. Basically, I've put them through everything a toddler would practically from the time they were little. If possible, take snakes that are not only handled a lot, but that don't get stressed out by it. Also, I always made sure that they'd been fed about 3 days before so they weren't looking around for food (might be longer for a ball).
Take them out, handle them while you're talking about that particular snake so you can also get a feel for how your snake is doing. Even the calmest snake can have an off day or a smell that you might not notice could be bothering them. By you handling them for a bit before anyone else gets the chance to touch, you can gauge the snake's mood. If they are getting stressed, upset, or scared, then don't allow the kids to touch that particular snake.
One thing I can tell you is it is a lot of fun to do and it's always neat to watch a child that might have been scared of snakes before, to come up and touch one once they see they are harmless. My best snake for programs has always been my blizzard corn who's calm but his other nice trait is that he's solid white. To many kids, he doesn't 'look' like a snake (I've been told this numerous times by them) so they are more willing to look, learn, and touch. If you have a morph which doesn't necessarily look like a snake (patterning, color, etc.) who has the temperament for this, that's one good option.

A while back my son had a science fair and they had some community outreach type stuff. One group had snakes. They had a 12-14 foot albino Burmese. Snake was great, seemed to really like the attention, but after about hours of kids playing with his tail and being, well kids, he decided enough was enough and musked everywhere. Of course my kid happened to be in the wrong place at the wrong time and took it right in the face, absolutely covered. Worst car ride ever!

Just goes to show even the best snake has a boiling point. Just make sure you know what yours' are!