You could do this easily with VLC, an open source, free media player and streaming server:
http://www.videolan.org/vlc/
There is a large document about streaming with VLC here:
http://www.videolan.org/doc/streamin...-howto-en.html
You'll want to read the section on streaming to Windows Media Player so you can setup a link that someone can click to open WMP and play your stream; for Mac users you'd probably want to use Quicktime to pickup an RTSP stream.
Hope this helps.
//edit: Also, this all assumes that you have a network connection already setup that's fast enough to support lots of users watching a video stream. You'd probably have to talk to the IT department at your school to get that part setup.
-Russ