Keith Barnes, 50, has been a teacher in the Haverford School District for 25 years.
Police say Barnes went to a bar on Saturday night to see the band Blackthorn with his two sons, a three-year-old and a four-year-old.
Barnes allegedly told police the three-year-old was fussing, so he took him outside and locked him in a car. A passerby spotted the child and called police.
Responding officers said it was a cold night as they tried to open the door. Barnes came out a short time later and unlocked the car for them.
Police say the child had been in the car for about 20 minutes.
When the police questioned Barnes, they say he didn't seem to grasp the gravity of the situation. Police say Barnes kept repeating that he's a musician and Blackthorn is a national act.
Police arrested Barnes and charged him with child endangerment and disorderly conduct. They say he had also been drinking and was close to the legal limit when he came outside to check on his son.
His wife answered the door on Wednesday at their home in Rutledge and said he wasn't home.
Barnes teaches technology courses at Haverford High School.
The district says he has been removed from the classroom pending the results of an internal investigation.