At the Glenwood Elementary School in Media, Pennsylvania, roughly 450 students interrupted their regular schedules one day this month for an unusual emergency drill.

Just after 1:30 p.m. on May 3, the entire student body practiced sheltering in place in the school’s gymnasium, then prepared to evacuate the campus by bus, under the watchful eye of the school’s superintendent, state police, and local first responders.

“Everyone took this seriously and it was reflected in how quickly they moved through the drill — two minutes to be sheltered in place and three minutes to be completely evacuated from the building,” Principal Eric Bucci told local reporters.

It wasn’t fears of natural disaster or terror attack that prompted the emergency drill. Instead, worries about a fossil fuel pipeline construction project nearby left the school district drafting emergency response plans and practicing safety protocols.

The school is one of dozens neighboring the route for Sunoco Logistics’ Mariner East 2 pipeline, now under construction and slated for completion this fall. At Glenwood Elementary, the Mariner East project will carry natural gas liquids like butane, propane, and ethane (used to make plastics) through a pipe under a road about 650 feet from the school’s playground — and some parents and safety experts are worried about the risks posed by leaks or other accidents.