Parent of multiple kids in schools mentioned.
Chance to get into Potomac, almost zero. Even for non entry years, there are still dozens applying and if not rejected, end up in the wait pool. They only go to the wait pool if someone decides to leave the school after 3/1 but before the 6/1 binding date. Chances of that are slim, and I don’t think they even take late applications. Even so, there will still be quite a few kids who went through the application cycle sitting in waitpool already.
Langley, will be easier. They also have full classes at all grades and more applicants than spots. But every year there are some kids who apply out to other school like at 4th or 7th. So there is perhaps more movement. But also, there are likely lots of kids in the wait pool the school would go to first over any late applicant.
Best advice, is to to apply through the normal admission cycle to have a chance. Even then, it can be 10-1 applicants to spots.