The case against Shaun Harrison, “dean of academy” at English High for more than four years, has become not so much about him but about how a possible dealer of drugs manages to fly under the radar screen of the police and the school department.

It’s a question Mayor Marty Walsh was also asking yesterday.

“It’s critical that, in addition to a criminal investigation, we take a thorough look at his employment within our public school system to ensure that we are taking the necessary steps to protect students throughout the city,” Walsh said in a statement.

Following Harrison’s arrest last week for the execution-style shooting of a 17-year-old, police searched his home and found two handguns, a rifle, ammunition and “trafficking weight” of cocaine and a large amount of marijuana.

Police Commissioner William Evans searched department records for complaints filed in the area and found plenty for Pompeii Street, where Harrison, 55, lived. In fact, in response to two complaints the drug unit was sent to put the building Harrison lived in under surveillance for several weeks last summer.

“But we didn’t see anything to take it to the next level,” he said.

Yet neighbors reported to Herald columnist Peter Gelzinis that there were kids coming and going at all hours, smoking weed and drinking beer on Harrison’s back porch in the summer and later in their cars parked in front.

The fact is lots of folks knew — many of them, no doubt, customers or neighbors who didn’t want to get involved or were discouraged by the inability of police to get a handle on it all, and yes even police who ought to know when they are being stonewalled.

And to those who insist drugs are a victimless crime — well, not on Pompeii Street.