Everything you send over the network is visible to anyone near you. Even if they do use MAC filtering, it doesn't help -- with no encryption on the network, you're broadcasting every packet in the clear over radio. Any wifi card capable of monitor mode can record everything you send. As André mentioned in comments, this is also potentially vulnerable to active attackers. MAC control does not protect users of the network from eavesdropping; it makes it slightly more annoying for an unauthorized user to connect to the network (this does have its uses; it discourages casual passersby from using the school network, and might fulfill some bureaucratic requirement that the school needs to follow), but that's it. At the scale of a large university, a pre-shared key would be no better (it might as well be completely public).
The only way to protect against that is to encrypt your traffic. A VPN can be a very effective way to do this; if you can connect to your school's VPN from the wifi network, this will encrypt your traffic over the air. Likewise, any site you visit using HTTPS will be encrypted, so an eavesdropper can't read your messages.
At a network level, the school has more options: the best one by far is to offer a secure network using WPA2-Enterprise, for which you log in to the network with school credentials, which are used to derive an encryption key specific to you (with WPA2-PSK, anyone else who knows the wifi password can still eavesdrop or perform active attacks; with Enterprise, they can't). For devices that don't support this (like game consoles), there's no really good option; what my school does is run a WPA2 secure network that everyone's recommended to use, and run an unencrypted MAC-controlled network for devices that just don't support the secure network. This requires some amount of infrastructure (nicer APs support it better, with one AP carrying both those networks), but it works well for security.