If the family attended by virtue of it being their automatic neighborhood school and they moved out at a grade that was not too crowded, then sometimes a principal can allow students to stay. This most often happens around 7th grade, especially if the student has been there for many years because 7th grade sometimes thins out with Academic Centers being an option for advanced middle schoolers.
It is rare and it is on a case by case basis, so it will be hard to predict years in advance if the school continues to be crowded in all grades.