A teen, a senior at Friendly, approached another teen -- a former student -- at about 9 p.m. and the two began to fight. The former student then pulled out a knife and began stabbing the teen.
There was a volleyball game going on inside the school on Allentown Road in Fort Washington at the time the fight broke out.
The student was taken to a local hospital, where his condition has been upgraded to stable.
The attacker was arrested a short time later at Fort Washington Hospital, police said. He was being treated for hand injuries he sustained during the fight.
The incident remains under investigation.