This is a cool little trick that works at my old high school (I've left there now as I'm 17) and may work everywhere else .
Step 1. Open up a word document on the school pc.
Step 2. Type something random into the document.
Step 3. Press CTRL+ALT+DEL and click log off (if there is not this option click wherever the option is on your computer.
Step 4. When it asks you to save the document, click cancel, this should stop the computer from logging off.
Step 5. Now on most schools it should show you as logged off, it worked in our school because i asked the teacher if i was logged on or not.
Now they shouldn't be able to take control or look at what you are doing on your computer. Have fun!
NOTE: THIS DOES NOT BYPASS WEBPAGE FILTERS AND MAY NOT WORK IN ALL SCHOOLS.