CHICAGO (CBS) — With less than three weeks before the start of class for Chicago Public Schools students, approximately 20,000 freshman seats are still vacant at district high schools.

At Frederick Douglass Academy High School, only one freshman has enrolled for the upcoming school year.

A University of Chicago study published in the Chicago Tribune shows, as of last month, not a single freshman had enrolled at Hirsch Metropolitan High School.

Researchers at the University of Chicago and the Federal Reserve Bank said part of the reason for the severe under-enrollment at some schools is a new online application process that allows graduating junior high students to rank their most desired schools.

That can leave some schools with more slots than interested students.

The problem is perhaps best exemplified by Douglass High School, where CPS officials expect only about a dozen freshmen to enroll by the time classes start. At Hirsch, about 50 freshman are expected to report to class.

Researchers believe as many as 7,000 of the 20,000 vacant seats will be filled, but they stressed low enrollment can make it difficult for officials to set up a curriculum for interested students.

Chicago Public Schools spokeswoman Emily Bolton said the study’s findings “show that families are highly engaged and making important decisions about their children’s education.”

“This research is providing us with an unprecedented look into families’ choices, programs, and trends, and we look forward to receiving additional research and insights after students enroll in the fall. CPS is committed to helping schools attract students, and this year launched a $10 million fund targeted specifically to help underenrolled schools offer more programming to students,” Bolton added.