The Alameda County Sheriff's Office says the dealer would use the hecticness of student drop-off and pick-up at McClymonds High School as cover.
As families were coming and going, the dealers would be selling to customers.
RELATED: Alameda Co. authorities recover more than 92 pounds of fentanyl in East Bay
Sheriff Greg Ahern said the dealers were not selling to the students.
"These type of drugs are for adult age people, not high school children," he said. "We don't want to allege they were focusing in on high school students."
"They want to blend in, they don't want an isolated spot," he said, explaining why they were possibly parked across from the school.
Investigators say the seizure totals 15 pounds of fentanyl, 1 kilogram of heroin and $139,000 in cash.
Four people were arrested.
"A use of an illicit drug is usually one tenth of a gram. You multiply that by pounds, you figure the number of people that would be able to have access to this illegal drug, causing death... it is a huge problem," Sheriff Ahern said.
His investigators say the drugs arrived here from another country through an organized network and were likely headed for San Francisco.
"We actually believe the cartel and the dealers are lacing their other drugs with fentanyl. They are lacing cocaine and their heroin with fentanyl to make it more addictive, which causes more problems in our communities," Ahern said.
The names of those arrested and the charges they face have not been released.