A security researchers two pesos:
Kid wants or is using a vpn to bypass content filter ACL's on the school's firewalls.
Environment: Based on provided information.
1+ Fortigate Firewall : Manufacturer-Fortinet
Kid owns it: If this is your laptop and your allowed to use the schools network then it's likely that the network your using is part of a DMZ. In other words, the Network guys don't care what you do on the Network and the content filters were put in place for either liability and or moral reasons. Its also likely that your egress traffic through the Firewalls are rate limited anyways, meaning that you can only use a limited amount of bandwidth. i.e.-1.5 Mbps for example. Point here is, most likely no one is paying attention so...
School owns the laptop:
If this is true then I'd assume that you have to login to a domain that the school setup. In other words, when you login to your laptop it might look something like " myschoolsucks\vpnkid ". If this true, then you might want at least ask what the policy is regarding the use of VPN's, and let the answer to that dictate your actions. No policy, no repercussions.
Fortinet products can actually be very advanced and shouldn't be disregarded as a low end Firewall. Also, if the network your using at school isn't a DMZ, but rather a "production" network, and I was in your position hypothetically. I would error on the side of caution because, and I'm speaking from experience. Your VPN, almost no matter how high the encryption values are, can absolutely be tracked, then packets tagged for collection, decrypted, recompiled and analyzed. Also, if the Network guys were smart would already be tracking and logging traffic from your device.
Ditch the VPN and buy a Logmein account, install it on a computer at home and just remote into your home computer when away...