The disguise thing I was thinking would have a true form and a fake form and the true form is a taller, more unkempt version of the usual prey students, whereas the fake looks like a normal student but slightly off, maybe with a smile or something. You would need to look out for suspicious behaviour, like a prey following you or trying to block your path and you would need to run before it shifts back into it's true form and devours you.
The harpy is more meant to be something really difficult to avoid, but fairly easy to escape from. It'd fly too high in the air to catch you normally, however when it gets close it swoops down to try and grab you, the swoop would be fast and tough to avoid, but would have an audio cue at the start and some lag at the end so you could escape, the main issue is potentially not noticing the harpy and trying to avoid it's swoop which is fast.
The ghost would have things that make it difficult but fairly easy at the same time. It would most likely wander floors and randomly fly through the ceiling, if it hears a sound it will immediately go to it and you would need a different strategy than just run and hide when being chased. However it would be quite slow and would make a constant sound, maybe a stereotypical moaning like in movies or a dripping like-sound with ectoplasm