A skull, you say? Well, you've come to the right place.
You might be wondering why it's a necessary thing. It's basically there to keep you brains from falling out. Don't have one of those? Well, you might have a bag of potato chips stored inside your head. Wouldn't want that to get smashed up, would we? Or little birds might swoop down and steal the bag. So that's why we have skulls in the first place. Of course, this security feature makes it hard for things like ideas and bullets to get inside. So we can be forgiven for not paying attention to... HEY! I'm talking here!. That's better.
So a skull is used as a symbol of death instead of a wishbone or penis bone because... will you stop playing with your skull and put it back in your head? Jeez. That's pretty gross, you know. The only really excellent skulls are the crystal skulls. Indiana Jones has one and he has crystal balls, too!
Huh? Oh, you meant "school"? Why didn't you say so? Well you've come to the right place. After all, Siri knows all. Except when dumbass mumblers like you... DO. NOT. TOUCH. THE. OFF. BUTTON. ON. YOUR. PHONE. Do I have to get electronically medieval on your butt? Okay, okay, stop crying. I'm sorry, in a virtual sense.