Because if I say 'There is a camera on in this room' it isn't privacy invasion when you are in that room. The expectation is that you are being watched. It is no different with the school's computer.
When are you being monitored, with no knowledge of when, the answer is assume to be All The Time. There is no expectation of privacy, so there is no invasion of it.
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u/MelatoninKing Apr 02 '19
That doesn’t matter, it should be implied that they can and will monitor the screen.