If you are worried about kids using the internet to do bad things, then filter the internet. Use something like SurfControl which runs essentially between the internet and your network and looks out for things you don't want your network users to be looking at. Since it's your internet, you should be able to dictate how it is used.
However, you said in your post that the laptops are basically theirs, so they should be the ones who decide how the laptops are used. Let them do what they want with them when they are at home. Using a filter that runs externally to the PCs is a great compromise. It gives them flexibility at home and the structure you need at school.