At the college where I work, in the comptuer labs, the instructor station has software installed that I can use to control the student's workstations. I can either just view a thumbnail of what they are doing, or I can take control of the workstation and use my mouse and keyboard to control their computer. I can also configue what sites they can and cannot go to, power the computer on and off and blank all of their screens. However, they all have to be on the same network and each of the workstations has the workstation version of the software installed on them.
I wouldn't think that just being on their wireless network would allow them to see your screen, but they could VERY easily track the URL's you are visiting. Since it is their network, you agree to their rules when you use it.