Homemade first aid kits are a gift that kids can help with. You can get some ideas here and here. I got three basic kits as wedding gifts and at the time I was thinking I only needed one. We ended up sticking one in the car and one in each bathroom and we still have the original cases and just refill as needed. Teachers usually end up getting supplies at their own expense so this could be really appreciated. You might want to give them a kit or get the items they need to replenish a kit they already have. Dollar stores supply a lot of basic items you could include like band-aids and tweezers.