Great ideas above. Must know:
Kids will try and fake anything to get out of class/school so learn how to assess well. The more they argue how sick they are the less they usually are!
Learn all you can about lice because there will be tons of it.
Teachers will usually freak out about a kid falling down or someone with lice quicker than they will about someone who may have a concussion or low blood sugar.
Teachers hate to deal with anything messy whether it's puke, poop, or snot so expect to have lots of clean ups. It's great to have extra clothes on hand for those times as well.
Be the child's advocate! Insist they go to the dr when needed and even play hardball and not let them return to school until seen by a dr if necessary. I've had kids with pneumonia, asthma, scabies, strep throat, ear infections, etc that I've had to literally force the parents to get them to the doctor and it's usually the ones that have a medical card and don't even have to pay for it. Help them get glasses, dental treatments, clothes, and school supplies if needed.
Expect to not get an uninterrupted lunch or a planning period like the teachers.
Some parents will freak out over ever little boo boo their child has (esp if you didn't notify them). You'll learn which ones to notify if their child crosses your threshold but it's better to just notify all parents if their child is going to come home with something they didn't go to school with like a cut, bruise, etc. If it's minor a note will do but if it's something the child may have to go to a dr about like a sprain, bump on the head, etc always just call.
Get on the good side of the secretary. They can be a big help when needed if they like you!
Teachers can be really annoying! There will be a few who get what you do but most don't.
Give hugs and smiles as needed. For some of these kids, school is the only place where they may feel safe, warm, fed, or even acknowledged.
Finally, always follow your gut whether it's about an injury that you feel needs checked by a dr or a possible abuse situation that you need to call Human Services about.
I'm sure there are tons more but this will get you started. Good luck-it can be a very rewarding job and a very trying one at times!