well i did something similar to what your wanting to do at my school, and got kicked out for it. heres how i did it at my school, although yours may be different, and there is probably an easier way that the way im going to show you. so..
first get a windows password cracking utility. this should be in the form of an iso file, write it to a disk and take it to school. put the disk in the cd drive, and turn the pc off. when it reboots, go into the bios section and select boot from disk or cd drive d: or whatever letter it is on your pc. once it boots from the disk, change the administrator password on your pc. now, before you select save and exit, unplug the network cable. this is to ensure that the server at your school does not try to overwrite the changes you just made. now save everything, turn off your pc, plug the cable back in, and it should start up and stop on the windows login screen. your pc will not be able to automatically connect to the schools network because you changed the administrator password to something the server does not recognize. now log into the administrator account with the password you selected. you may now surf the web freely without being under the restrictive access of your schools network or the pc your using.
i hope i helped. this method has worked for me several times, so theres a good chance it will work for you. and if there are any easier methods that anyone knows about, please post them.