Close a portal to another dimension that is in the area.

Or send up to 200 HP worth of creatures in the area that are from another dimension back to the dimension from whence they came.
(Pick a creature you want to expel.
If it is from another dimension, and has less HP than your Expel has left, the Headmaster will subtract its HP from the total left for this spell, and expel it.
Then you may repeat this with another target if the spell has not been used up.)

Expulsion Effort:
If a creature from another dimension has too much HP for it to be expelled, it uses up all remaining HP for that Expel, but still progresses toward being expelled.
Instead of being immediately expelled, it is enchanted with lingering expulsion magic.
Add the HP left for that Expel to a running total of expulsion effort for that creature. If its HP is ever below the amount of expulsion effort on it, it is immediately sent back to the dimension it came from.
Subsequent castings of Expel on that target strengthen and add to this magic.
When a creature completes a long rest, it breaks this enchantment and removes all expulsion effort from itself.