ED visa when leaving Thailand

My son had a 1 year ED visa extension a while ago.

For different reasons, it happens that the extension was ending the 31st of May while the end of the school year is the 20th of June.

His school (an international school ) made a new letter. We went back the the immigration office and they extended again the visa until the 30 of June.

The question is... school is ending the 20th of June... we plan to leave Thailand to go to Cambodia (holidays) , request an SETV there to come back in thailand and wait for the new school year to apply for another ED visa.

Does he have to make the cancellation process before leaving thailand (planing on the 25th) even though the school already mentioned that school is ending the 20th but immigration gave until 30th?