If by "signal" you mean the cell signal, then yes you can install cell phone blocking devices (which raises the question, what do you do in an emergency?). If you mean wifi, then you make the devices useless as they won't be able to communicate with the school servers, etc.

You could block certain ports from accessing the Internet (e.g. IM apps), and you could block certain websites (e.g. FaceBook) via a proxy server, but both can be worked around in various ways.