March is school enrollment season. Overflow happens every year, especially to those popular schools.
Many parents worry about overflows. It's reasonable because they spent a big fortune buying a house to attend that "popular" school. If their kids are overflowed to other schools, it may take a year for them to get back to the home school.
However, overflow is not always that bad. Usually, students are overflowed to the nearby schools within the same school district. Those schools' performance may not have big difference. Sometimes, students may be overflowed to a "better" school. For example, in Palo Alto Unified School district, Addison's students may be overflowed to Duveneck or Walter Hayes. Those three schools are all very good elementary schools.
If your kid is overflowed to a neighborhood school and you'd like to keep him/her there, you can submit an intra-district transfer request. Usually, your kid have high priority over the new registrations.
But the policy may vary in different school districts. The safest way is to call the district office to confirm.