A 15-year-old boy slashed a fellow student inside a Queens public high school on Monday morning – the third day of classes in the Big Apple, cops said. 

The victim, a 16-year-old boy, was inside Newtown High School on 90th Street near 48th Avenue in Elmhurst when the younger teen pulled out a knife and sliced him on the right arm around 11:40 a.m., cops said. 

The 15-year-old boy ran out of the school, but cops caught up to him and cuffed him in the rear of a building down the block, police said. 

Cops recovered a knife, authorities said.

Charges against the teen were pending.

The injured boy was taken to Elmhurst Hospital Center, where he was listed in stable condition. 

The motive for the student-on-student violence was not immediately known.