If you plan to leave before the stamp date then I'd just forget about it. You could ask at a local immigration office if they would adjust the stamp, unlikely tho since it's a land crossing.
You can cancel it yourself, get a letter from the school stating your last day of classes and take it to immigration. Immigration will adjust your permission to stay stamp, this can be done with a date in the future to give you time to leave.
In your cover letter state that your clients are abroad and you don't/won't take any Thai based clients. You might need to supply a couple passport sized photos, data page of your passport and once in Vietnam a photo of the Vietnam visa you used to enter.