Chris K,
It's easy enough for the employers to say "Hey, school kids are going to be here for half an hour, close down whatever you're working on and show them a pre-rendered video of something from last year"
SSS,
Don't be intimidated, but expect to be turned down.
If you're lucky and you do get a tour, make sure you have a teacher (or parents, etc) that can escort you around, and try to organize some set structure to the tour with whoever you organize it with, that way when you get there, you and the company know exactly what you're going to and meant to be doing.
Jess.