Once, perhaps unfairly, they fit a Mayberry image. School-based police officers were seen as friendly if a bit over-the-hill, a touch out of shape and counting the days to retirement as they watched children head from the lunchroom to recess. If anything serious happened, they were to lock the building and wait for help.

But the string of mass killings at schools over the past 15 years has changed all that, adding urgent new duties and risks to the job and drawing in younger and often burlier officers. Out of hard experience has come a major rethinking of tactics in a crisis.

At the annual meeting of the National Association of School Resource Officers in La Quinta, Calif., last week, school-based officers crowded into multiple sessions on a theme hardly pondered by earlier generations, “the active shooter,” and discussed the aggressive responses now being recommended by security experts and federal officials.

"You’re not just Officer Friendly anymore," said Kevin Quinn, the association’s president and a school police officer from Arizona.