An 18-year-old was stabbed outside a Lower Manhattan high school Friday afternoon, police said.

The teen was stabbed once in the back near 198 Forsyth St., where Cascades High School is located, according to police.

He was rushed to Bellevue Hospital and is expected to survive, cops said.

The attacker, who is believed to be a 20-year-old man, ran off in an unknown direction, cops said.

It was unclear if the victim was a student at the high school or what sparked the violence.