Authorities were using canines, the town board of education says.
Hackettstown High School students were sheltering in place on Wednesday morning due to a drug sweep, town police report.
It was a planned event, police said in a Nixle alert.
"The children are safe and in no danger," police said.
The canine sweep was in conjunction with the Warren County Prosecutor's Office, police added on Facebook, citing the town board of education.
"Please do not attempt to enter school grounds while this planned narcotics sweep is taking place," the board requested.
The message will be updated when the operation is complete, the board said.
"We thank you in advance for your cooperation in partnering with us," the board said in the police Facebook post.
Tony Rhodin may be reached at arhodin@lehighvalleylive.com. Follow him on Twitter @TonyRhodin. Find lehighvalleylive.com on Facebook.