if its been in a school then it will have plenty of hours on it, they’re pretty much left on all day every day (the the high milage £50 jobbers I buy are from schools).
Just switch it on and plug things into it – the inputs will dicate what you can play through it. Looks like they just have VGA to play from a computer, S-Video and component video which would normally feed from a DVD player but not HDMI or scart which might rule out some digi-boxes . You can’t break it by pressing the buttons. If its ceiling mounted you’ll need to go through the menus to flip the image round so that you can sit on a table for portable use.
If you’re planning to use it outside then you’ll need a lot of dark for the image to be any good. The smaller you make the image the better too.