First of all, it's not a good idea to use the school computer, but you're looking for a way to unlock it, not the answer, right? Let me tell you. This is a fairly easy method, so you can do it in no time. First, press the three dots on the top left. Then, press "More Tools" and press "Task Manager" from among them, and an extended function will appear in the middle of the screen. After pressing "End Process" below, you are now free.