They will be most interested in the hands-on stuff you take with you. Explain your rules first, this is what you can touch, this is what you can't touch. The staff will help you. Stay away from blood and guts, make nurses and doctors seem as friendly as possible because I guarantee you will have kids in there that the moronic parents have used the doctor as a threat to behave with. "If you don't stop that right now I'm taking you to the doctor for shot!!!" Then they wonder why they kids are hysterical on the way to the office for a check-up or shots!
Pick a volunteer, start with your own if possible and as soon as the others see her getting her heart listened to, or the "honor" of having the BP cuff on, they will follow suite. Just make sure they follow the rules and don't let anyone including your own break a rule or the other herd bound little suckers will be on you like a pack of dogs on a three-legged cat!
Washing hands, explain how long and then use the in-class sink if they have one, and have someone show you how well they listened. Take some colorful band-aides with you and use them as prizes for correct answers and try and make sure everyone gets one.
You don't have to take long, but I promise you time will fly and you'll wish you had another hour to talk!
Really push being the good guy though!!! Nurses and doctors are your friends!