You need a few things. First you need a login or remote control mechanism. Perhaps something like Microsoft Remote Desktop, VNC, GotomyPC or whatever you decide to use. Then second you'll need to open up the inbound ports in the firewall specific to that software. To do this you'll need administrative access to the router and permission to make the changes, which I'm assuming you have.
Thirdly, remember that allowing remote access like this raises all sorts of security implications and some methods are more secure than others. You might like to consider a 'proper' VPN or SSL tunnel approach as I'm sure the school will take a dim view if the network gets compromised from the outside