No it’s quite normal for tourists in south east Asia to travel for months without plans so they won’t bother.
If your travel patterns look out of the ordinary they may start asking questions and/or requesting proof until they are satisfied or you are sent back to where you came from.
you have to get the school to write you a cancellation letter which you then take to immigration office to cancel and get a new stamp with the date you must leave by.
So its important that you get the school to date it for the time you leave