Declining enrollments mean tax savings for taxpayers if the buildings are no longer in use, or the number of teachers is down. 

Some cities close their lower enrollment schools-some sell them. And some convert them to charter schools.

A few titles under Google:
*Closing Schools to Improve Student Achievement: What the Research and Researchers Say
*Declining Enrollment Dictates Closings | Twin Cities Daily Planet
*Closing Public Schools in Philadelphia 
*In rural school districts, declining enrollments — and hard choices ..
*Declining Enrollment, Part 3: What Happens When A Town Closes Its Schools

Post note: That, interestingly enough, is one the highest costly reasons taxpayers want illegal immigration dealt with, the Per Pupil costs.