Sign up for Chalkbeat Philadelphia’s free newsletter to keep up with the city’s public school system.Philadelphia families and educators have been fighting for months against the district’s proposal to close 18 schools. They worry about taking resources away from struggling neighborhoods and limiting the choices that families have long been promised. But there’s a basic problem driving the closures that isn’t up for debate: There aren’t enough students to fill the city’s schools. And the situation is only expected to get worse.Over the past decade, enrollment in Philadelphia’s public schools has declined by more than 17,000 students. That’s largely a result of declining birth rates and a sharp increase in the number of students attending cyber charter schools.At the same time, some neighborhoods’ population has skyrocketed, driven largely by arriving immigrants. That means even as the number of empty seats in some schools has grown into the hundreds, others have become overcrowded. While the district’s overall enrollment has shrunk nearly every year, the portion of English language learners has climbed.This school year, around 117,000 students attend Philadelphia’s district and alternative schools — a 13% decline from a decade ago. Over the next decade, the state projects enrollment will likely fall by another 10%.The Board of Education has yet to approve the school closure proposal. It plans to host a town hall on Thursday to hear more feedback. No board member has indicated support for the proposal yet. But board members have acknowledged that the city’s declining school enrollment must be addressed. The district already doesn’t have enough funding to meet what the state has determined it would require to adequately educate its students. Keeping buildings open with fewer and fewer kids is a cost it may not be able to bear.“We cannot ignore enrollment shifts,” Board President Reginald Streater wrote in an op-ed last week. “We cannot wait until financial instability forces decisions upon us. But we also cannot allow financial pressure to take priority over our mission.”School enrollment decline unlikely to reverse Philadelphia is far from the only district in this position. Birthrates are declining nationwide, and researchers predict nationwide public school enrollment will fall by 5% between 2022 and 2031. In Philly, that decline is made more severe due to the rising popularity of cyber charters. In 2014, only around 550 students attended cyber charters. Last school year, more than 14,000 students did, despite concerns from some officials and education groups about the quality of education they provide. State officials predict the Philadelphia district won’t be able to recover the thousands of children it has lost over the past decade. Instead, the Pennsylvania Department of Education’s enrollment projection model, which factors in trends in birth rates and student progression through grades, predicts Philly’s enrollment will decline by another roughly 10,000 students by 2034-35.Superintendent Tony Watlington has said he does not want to close schools. But he’s forging ahead because the proposal would improve opportunities for the students who remain in district schools, he told the Board of Education last month. The plan would also mean no students go to class in buildings rated as poor or unsatisfactory, according to Watlington.Watlington has said he is committed to keeping and attracting students to the district. But if enrollment continues to decline, he said school closures could come up again.“You just can’t lose your student population — and for whatever reason, people leave — and then expect to maintain the same level of building footprint,” Watlington said.Rebecca Redelmeier is a reporter at Chalkbeat Philadelphia. She writes about public schools, early childhood education, and issues that affect students, families, and educators across Philadelphia. Contact Rebecca at rredelmeier@chalkbeat.org.