Not always so, they change this rule yearly.
When my daughter started school in Calvert they denied all transfers and I had to switch daycares. The year following some were allowed. I know they've done this in Charles also.
What is it you're trying to do? Stay in the same county or outside the county?
Actually, it depends on which school and which county. From what I understand, Calvert tends to be less likely to allow school shuffling of any sort. Other counties decide on a case by case basis depending on how crowded the school is and the reason behind the desire to change.
Looking at the original poster's other thread...she doesn't even live here yet.
RRANEA, you might want to consider that if you set up as a family child care provider on a given school district, but seek to send your own children to school in a different district, that's going to be a big red flag to your potential clients. They aren't going to seek care for their school-age children in a child care where you won't even send your children to the local school. Plus, chances are, the school system will deny your request for a change of schools. The number one reason for changing schools is day care arrangements. Since that will obviously not be the case for you and the schools will figure that out quickly, they will most likely deny your request.
The only time you can change schools just because you don't like the school you are zoned for is if that school has failed certain standards under the federal "no child left behind" law. A few years ago, there were 3 schools in St. Mary's County that had failed and there was a big write up about how parents could pull those kids out of those schools and the school system had to let them enroll elsewhere. I believe, however, that all of those schools have redeemed themselves and brought up their ratings so that this no longer applies.