If someone moves out of a district, there isn’t any responsibility to ‘notify’ the school (unless the move is during the school year).
Again, these types of choices are a lot less complicated than many people think. Choosing to live abroad and not having educational credits upon return just means the students need to complete the remaining credits necessary to graduate.
Whether this is a good idea for these two students isn’t something that I think any of us can opine about knowledgeably. Their parent(s) will have a much better idea of the situation, the possible solutions and the two children involved than any of us.