A middle school principal was charged with dealing crystal methamphetamine after police found the drug in his school office.

John Acerra, 50, of Allentown, was arrested Tuesday in his office at Nitschmann Middle School in Bethlehem, where police said they found meth on his desk.

Police said they began investigating Acerra in early February after an informant told them that the principal was using and distributing the drug.

There was no indication that Acerra sold the drug to students, but Acerra did allegedly sell the drug from his school office after hours and on weekends, said Dennis Mihalopoulos, an agent with the Drug Enforcement Agency.

On Thursday, police watched Acerra sell a small amount of meth to a customer in a store parking lot, according to court documents. Police stopped the buyer, who told officers he had been to Acerra's home 10 to 15 times over the past three months, officials said.

Police then arranged for an informant to buy $200 worth of meth from Acerra on Saturday in the parking lot of an Allentown drug store, according to court documents.

On Tuesday, police set up another $200 deal with Acerra and the informant, who wore a listening device as he began conducting the transaction inside the principal's office, authorities said. Acerra did not have enough meth to sell to the informant, and he and the informant arranged to meet later that night to complete the buy, Mihalopoulos said Wednesday.

After the informant left the building, police entered Acerra's office and found him sitting at his desk with a bag of meth next to a glass tube with meth residue and burn marks on it. Also on the desk was the marked money the informant used to purchase the drug, court documents said.

Acerra has an unlisted phone number and it wasn't clear if he had an attorney. He was arraigned on felony drug charges and sent to Lehigh County Prison, with bail set at $200,000.

Bethlehem Area School District Superintendent Joseph Lewis did not immediately return a phone call from The Associated Press on Wednesday. District officials were planning to hold a news conference Wednesday afternoon.

Acerra became Nitschmann's principal in 2000 with a salary of $80,467 a year. Before that, he was the school's director of instruction and curriculum.