Investigators said the teacher asked the boy to turn out his pockets after another student reported being shown cash, The Philadelphia Inquirer reported. The boy had eight bags of a white substance looking like crack.
Police searched the classroom at the Thomas Mifflin School with drug-sniffing dogs.
Chief Inspector Cynthia Dorsey said there have been occasional incidents of elementary school students bringing drugs to class.
"It's not common, but it does happen," she said. "I have never heard of a pre-K child."
The boy's uncle was in custody and being questioned by police, KYW-TV reported.
The boy was taken to St. Christopher Hospital to determine if he had ingested any drugs. He does not face any charges.
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