Yeah, in this lawsuit happy world you have to be very careful . . . I wouldn't let the kids handle the snakes unless you have hand sanitizer on hand AND have them wash. Maybe a little paranoid, however it is a big deal. And you should explain about salmonella, if the kids understand why you want them to wash up they are more likely to do actually do it. That said . . . I wouldn't worry too much about 8th graders being bratty, don't let 'em bug you, at that age kids just love to be annoying. I always try to remember I was that age once . . . LOL . . . and if they are disrepectful you can always tell them to cool out or you can take your snakes and go. However, to start off you might try to relate to them on their level, explain how you got interested in snakes, why you like to keep them, etc. One thing they will probably be fascinated by is how snakes eat, you might take some pics w/you. But you might pass them around so those who are easily grossed out don't have to look. I don't know . . . some ideas. You've got some other great ideas too. I love working with kids!!! Relax and have fun!!
Oh, and 1 more thing . . . I've found this really helps when I give a presentation for a class. Have an outline or notes with the key points you want to hit, so you don't forget. I know that when I get up in front of a group things I meant to say seem to slip out of my mind. After you have an outline or notes on what you want to talk about go through it and time yourself. You don't want to discover you only have 15 minutes of material to fill a 45 minute class after you are already there. Or MORE material than you can cover in the allotted time. I know that sounds pretty basic, but it can make a huge difference once you are up in front of a class.