The number of homeless students nationwide has more than doubled in 15 years to a new high of 1.5 million in the 2017-18 school year, according to data released from the National Center for Homeless Education last week.

For context, that means there are as many homeless students in America as there are people living in New Hampshire — if not more.

The big question is why — which is complicated by the fact that student homelessness is measured differently than homelessness among the adult population, and still considered a vast undercount.

The National Center for Homeless Education, run out of the University of North Carolina at Greensboro and supported by the U.S. Department of Education, has been examining and analyzing federal data on student homelessness since the 2004-05 academic year. And since that academic year, the number of homeless students nationwide has more than doubled — and that’s likely an underestimate of the actual problem — according to the New York Times. That has advocates and experts worried.

“The record number of children and youth experiencing homelessness nationwide is alarming,” said Barbara Duffield, executive director of SchoolHouse Connection, a nonprofit that focuses on homeless youth, in a statement. “But for many of these children and youth, public schools are their best — and often only — source of support.”

Since the 2015-2016 school year, student homelessness has increased by 15%, from 1.3 million to 1.5 million, according to the report. The vast majority of those homeless students are “doubled up,” or staying in a home with another family, although the rate of unsheltered homelessness among students has also risen a shocking 137% since the 2015-2016 school year to encompass 102,000 kids.

Student homelessness gained most in states with a combination of yawning wealth gaps, stunted minimum wages, devastating natural disasters, and affordable housing shortages, and no single cause can account for the record-high population. While homelessness is measured differently by the U.S. Department of Housing and Urban Development —that agency doesn’t include multiple families squeezing into one housing unit, and therefore counted only 568,000 homeless people last year — the general consensus is that homelessness has gained in recent years due to a shortage of affordable rental housing and a weakening safety net for the poor.

On the local level, the largest rise in student homelessness has been in Texas, where the number of homeless students has doubled in three years to reach more than 231,000 people. Other states and territories that have experienced large spikes in student homelessness since the 2015-2016 school year include Florida, Rhode Island, Puerto Rico, Connecticut, Pennsylvania, and Montana, according to the report.

“We’re not moving the needle as much as we could because of the lack of affordable housing,” Karen Barber, the director of federal programs for the Santa Rosa County, Florida school district, told the New York Times. “That really is the biggest issue.”