The closure came after someone found a body near school grounds early Tuesday morning.
Police say two males were in cars Monday night in the area, and only one of them was left alive.
The man killed was in his 60s and often frequented the Darby neighborhood.
Police believe the two men knew each other.
A car was towed away from the scene.
Police are waiting for the medical examiner to determine how the man died.
Penn Wood Middle School will now be closed all day.— Penn Wood Middle (@PennWoodMiddle) October 24, 2017
Students arrived to find police crime scene tape surrounding part of the school. They were then sent home.
Detectives were on the 100 block of Summit Street. They were called out to investigate after someone found a body nearby.
As day broke the road remained shut down as detectives searched for evidence.
Initially the middle school was going to open on a two-hour delay, but then administrators decided to close the school to students for the day.
Teachers were told to still report to school.
Some people were surprised to hear about the police activity in the area.
"To hear something like this, where... oh my God," said resident Tina Hayes, "on a quiet street. It's basically a quiet street. It's frightening."
There are surveillance cameras installed on the outside of the school, but it is not clear if they recorded anything that might be of use to investigators.
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