WASHINGTON — After the first day of school at Mark T. Sheehan High School in Wallingford, Conn., Mackenzie Bushey, a 15-year-old junior, came home upset that a teacher enforced a no-cellphones policy by confiscating students’ phones before class. She needed her cell, Mackenzie told her family last month, to notify police should a gunman attack her school.

And also, she said, “to say my final goodbye to you.”

Mackenzie’s mother, Brenda Bushey, blames her daughter’s fears on monthly active-shooter drills at Sheehan High. “I understand they’re trying to think about the children’s best interests,’’ Ms. Bushey said in an interview. “But you can’t help but think of how it’s affecting them.”

Nearly every American public school now conducts lockdown drills — 96 percent in 2015 and 2016 — according to the Education Department’s National Center for Education Statistics. Law enforcement officials and many school administrators say they are crucial for preparing and safeguarding students, but methods vary widely and now include drills that child trauma experts say do little more than terrify already anxious children.

“A whole new cottage industry has emerged where people who don’t know anything about kids are jumping in and adapting protocols for groups like police officers or people preparing for combat,” said Bruce D. Perry, founder of the ChildTrauma Academy, whose clinical team assists maltreated and traumatized children through counseling, research and education. As a result, Dr. Perry said in an interview, “The number of developmentally uninformed, child-uninformed and completely stupid ideas is mind-numbing.”