LCD, glass or plastic?

I am just wondering that the stupid guys at school who couldn’t give a rats bum about other property, They touch the LCD computerscreens all the time and i mean ‘press really hard’, and when they do the screen discolours and goes dark around there fingers, you know what i mean, so anyhoo, is it actuall plastic or glass that covers the LCD screen on computers, because if it is glass, it must be thin!!!