First, this isn't really the right forum, but whatever.
Alright, let's take the worst possible scenarios here.
(LOCAL) SOFTWARE BASED MONITORING:
Assuming the school decided to go all out on the software front, and managed to get software that conceals itself (perhaps via rootkit technology which hasn't hit the press yet), then the solution is simple: download a secure wipe liveCD (Darik's Boot and Nuke is pretty good) on a trusted computer, make sure you check the MD5 sums (to ensure against both someone hijacking your download (possible, but takes a _lot_ of effort, and thus unlikely) and against damaged downloads/burns (actually surprisingly common)); and use it to wipe the hard drive clean. Then get some new MBP CDs from a trusted source (Apple Store, or a trusted friend with a non-school bought MBP), and install a new version of the OS. And don't install any other software the school provides.
After this is done, the only way the school is going to put software on your computer is with your permission, or via an exploit, and there aren't a whole lot of those for OS X (though they _do_ exist).
NETWORK BASED MONITORING:
This can be more or less complicated, depending on the school's sophistication. The best middle ground between security and ease of use I think is Tor. It's available at
http://tor.eff.org. The details are all there, but basically Tor functions as a proxy, and instead of letting the school's servers connect you to websites, instead it uses the schools servers to forward your internet traffic to another, random, Tor-participating server, which in turn passes it to a second, random, Tor-participating server, which in turn passes it to a _third_, random, Tor-participating server, which then passes on your traffic to the actual destination. And then all the way back again. This is done in such a way that no server can tell both the originating and destination computer (everything's encrypted). So what this means is the school knows you're talking to <random computer #1>, but not what you're saying to them.
Cheers.