Tunneling isn't illegal, as long as you own the machines being used. Otherwise that comes under the general hacking other peoples machines for your own usage.
Though the system admins are likely to not be happy when they find the tunnel, and you might get in trouble for it.
VPNing home is another option, which should work. Though schools often don't like people bypassing firewalls, as they tend to have them in place for a reason (if only to spoil the fun and prevent gaming).