Paul Stein, 34, taught physics, chemistry and engineering at Carver High School in North Philadelphia.
Stein was arrested on drug and gun charges last Wednesday night, allegedly selling marijuana to an undercover officer outside Cheltenham High School.
In fact, police say he sold marijuana to undercover officers twice.
When he was arrested, police say Stein was armed with a handgun.
Police searched Stein's home and found another pound of pot, 5 guns, a bullet proof vest, and $16,000 cash.
He posted $150,000 cash bail to get out of jail Friday.
Stein wasn't home at his second floor apartment in Northeast Philadelphia today.
News of his arrest spread quickly at his school.
"He was real fun, real fun. He was real nice. He was a real cool teacher," said 10th grader Courtney Goodwin.
"Someone told me on Facebook - they posted "Free Stein" - then he posted a link and I saw it. It was crazy, I showed everybody," said 11th grader Jared Mayo.
Stein resigned from his teaching position on Tuesday and faces a preliminary hearing on multiple charges early next month.