I'm guessing your school pc has internet explorer, so here's what you do:
Click on Tools > Internet Options > Advanced tab and then the Reset button. Voila. Reset Internet Explorer (i.e. X it out and open it again) for the settings to take effect.
But if your school is like mine, got smart to this and has removed the Advanced tab, Go here:
Click the Fix It button and Run the program (save it if you want, that way if they catch onto THIS too at least you can cheat) and follow the prompts. A window will appear behind it asking if you want to reset the settings, click reset. Reset Internet Explorer if it is running or the settings will not take effect.
If you can't access this from school, do it from home and tranfer it via email, USB stick, or a non blocked upload site (like Mediafire).
Now go watch YouTube!