What exactly do you mean by how do you secure them? Do you mean how do you keep them from being stolen? Or do you simply mean how to do secure them from getting viruses? Honestly, I don't think you are going to be able to completely secure them from viruses. In fact, no one can completely secure the computers from viruses. If a software company could promise that, they would be bigger than Google ;-)
Your best bet is to do like many public libraries do. Create a user account for them to login to and only you have administrator access. Then you limit what each user can do. For example, you can prevent downloads. You can prevent them from installing programs. Etc.
I'm not sure if you want to do much limiting though. Students sometimes need to do all of the things you may want to limit. So maybe you just need to have a conversation with them about being careful about what they do.