A two-alarm fire at prestigious Regis High School early today sent thick black smoke billowing throughout the Upper East Side neighborhood.

The blaze started just after midnight and went to two alarms at about 12:30 a.m. at the tuition-free, boys-only Catholic school at 55 E. 84th St.

There were no injuries, authorities said.

“The smell was absolutely putrid, acrid. . .” said Ken Edlow, 70, who saw the inferno from his apartment.