HVAC School isn’t your typical training program — it doesn’t follow a set curriculum, nor does it operate out of a classroom.

The study found that 25- to 34-year-old graduate degree holders earned a median wage of $69,700, while bachelor’s degree holders earned $59,600. High school graduates earned $36,600.

Last month, graduates of the University of Southern California’s online master’s in social work sued the school, claiming its program, in partnership with 2U, is a degree mill.

“This year, our college jumped eight places in U.S. News and World Report ’s online graduate education rankings, which places our programs within the top 20 of more than 330 programs across the nation ...