Is there anything other than the rules that stops students leaving school grounds without permission? How severe is the punishment for doing so? And also the punishment for skipping classes with or without leaving the castle grounds?

  

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There is also a magical barrier that only the HM can take down. That’s why even staff have to let him know in advance that they need to leave.
https://charmingrp.com/misc.php?page=hogwarts

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   Aldous Crouch

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Yeah, I wondered about the barrier, but wasn't sure if it blocked students and staff, for example if a seeker is on an extended excursion will they eventually reach the barrier and collide with it.

  

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Besides the barrier, I believe none of the tunnels out exist yet either! It's very improbable students would figure out how to get off grounds if even professors need Phinny's powers to leave. I suppose you could get into the forest, but that just screams Death.

It might vary by professor, but Mason for example, the first time he found out someone was skipping for no reason, would give them a very unpleasant detention with manual labor and dragon dung. The second time it would be brought up to the HoH and then up the chain from there. I would also assume house matrons play a role in making sure students are attending classes, unless they are ill!

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   Christabel Daphnel

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