Even if the school didn't limit connections, you'd be a fool to not use a router for the same reason it's risky to use open wifi at Mickey D's or Starbucks. You're sharing that network w/ others! But by using your own router, you place your devices behind the protection of your own router's firewall and secured wireless AP. That alone is enough to warrant a router, even if you only had ONE device!
In fact, just like anytime you use open wifi, you should probably be using a VPN for internet access as well, thus protecting your traffic from eavesdropping. That could either be a paid VPN service provider (pretty cheap when you consider what ppl pay for tuition these days), or perhaps to your own VPN server back home (so it’s free).