Boy, that's a hot topic for sure! Unless you have dedicated equipment, there's really no way to be certain if cross-contamination is occurring. Sconeboy's got a few friends with severe allergies, so I've learned a thing or two over the years. If it's a tree nut AND peanut allergy, on whatever you buy, the package needs to say "tree nut and peanut free". Always check with the parents and teachers first, to see if they are okay with the products and home equipment you are using. And lastly, here's a great resource for peanut-free, tree nut-free, dairy-free recipes: http://www.amazon.com/Divvies-Bakery-Cookbook-Dairy-Delicious/dp/0312605285 Wait till your class has some gluten-free kids - that's a whole other ball game!

Just found this link, hope it helps you!!
http://kidshealth.org/teen/food_fitness/nutrition/nut_allergy.html

You need to check labels really carefully for things such as "processed in a facility that also produces nuts", etc..
You're a trooper for volunteering to make muffins for kids with known nut allergies, I'd be a nervous wreck and would be on the hunt for an epi pen to take a long!!