Let’s look at this from several perspectives.
What if You Change Your Mind About Reenlisting?
I have seen this numerous times. Soldiers decide they will ETS and decide not to go to the promotion board, schooling, etc. Then they reenlist and find themselves at a significant disadvantage. I guess the real question is when does your reenlistment window close. Is there a date that you are no longer eligible to reenlist?
Cancelling the School Slot
While this may be an option slots are usually tied to a name, so unit leadership can be resistant to changing or cancelling the school slot.
The only disadvantage that occurs is that this takes away valuable resources that could go to another Soldier and it reduces the time in which you could focus on ETSing.
Is cancelling possible? Yes. Is it the wise decision? Only you can answer that.