We had the pediatrician write us a prescription for a portable one and have never had any issues with it. It is smaller than a purse and hardly weighs anything. It plugs into the car cigarette lighter, the elec. outlet with a regular charger, or you can just plug it directly into the outlet. The battery pack is very easy to use and I take it everywhere. I often forget I have it hanging with my purse straps.
It is a Pari. I am adding a link, the yellow and black part is about the size of my hand. It does not hang over the edges when I place in on my hand flat. It is that small. Smaller than an average ladies wallet.
http://pari.com/pdd/trek-s.htm
Here is a picture of the battery pack by itself. You know how big the plugs for car cigarette lighters are, this gives a good example of how small it is.
By getting a portable one you can easily carry it back and forth daily. Just hang it up in the classroom where it is out of reach of little hands and then pick it up each day. The carrying bag has a zipper part that is perfect for putting the med vials in.