One possibility is that your phone is determining that the data connection is stronger/faster than the wireless connection and is purposefully changing to the more powerful connection. To test if this is the case, you can disable Cellular data on your phone when you want it to be connected only to the wifi. Not an optimal solution, but one that guarantees your data plan is saved.
Disabling the Cellular Data is under Settings > General > Cellular > Cellular Data and disable the option.
If the phone continues to disconnect with Cellular Data disabled, it's possible that your phone is experiencing the access points in the area having a channel conflict either with each other or with other wireless broadcast points in the area. This can happen if someone has one of the portable wireless access points broadcasting or when the access points in the area are not properly configured (or are on automatic channel select). There is also a possibility that the access point nearest you is on channel 1 and iOS devices really tend to prefer channel 6 or especially 11.
I work for a company that provides public wireless access at various locations and have experienced all of the above possibilities at various points with customers on iOS devices. If you feel like any of the above possibilities are what's occurring, you'll have to contact your college's wifi tech support or IT ops department because the