Yes it can be done. I don't think that the school will want to give you remote access to their full network, so a simple VPN solution where you log in to their router probably won't be allowed. I would look at something like this:
http://www.****.biz/Pres2005CD.htm
It makes a secure connection from the machine on the school network to the Talk2M Open VPN server, it doesn't matter what the IP address is of the school router. You then use software on your computer to make another Open VPN connection to the Talk2M Open VPN server and the server joins together the two ends of the conversation. Only downside is price for the ****, you are talking at least 1000euro.
Bryan