Heya
I guess it depends on what you mean by 90% of the web
If they're simply blocking ports then you could, in principal, make a tunneling solution to allow your connection through their security. Given that the Net7 proxy (most likely) only connects through specific ports, you'll need a way for that communication to pass through the school WiFi to those ports on the Net7 server. By using SSH with tunneling (PuTTY, for example) on a sanctioned port (22 might be blocked) you can have the Net7 proxy connect through another local proxy that, in turn, translates the port communication through a tunnel to an external SSH server that does the actual communicating with the Net7 servers. The solution can be pretty messy to set up (not to mention you could have yourself quite some lag) and does require an external shell account of sorts (if you've got a Linux box or another SSH server in your own home this does not have to be difficult) for the SSH tunnel.
On the other hand, if the security blocks specific domains and IP addresses you might have a much harder time trying to punch through. I wouldn't expect them to allow access to a very limited set of sites, but you never know. In this case you'd either need the IP address and stuff from Net7 to be accessible or, failing that, a proxy server on an accessible IP address you could connect through.
Anyway, I might be grasping at straws here. Best I've got right now, though