At 1:30 p.m. on April 28, inside the McKinley Avenue Elementary School in Stafford Township, hundreds of youngsters and their teachers crammed into the hallways near the school’s gymnasium, awaiting the signal from Interim Principal Hope Scherlin to make an ambush.

Inside the gym, Susan Malmstrom was wrapping up another one of the tens of thousands of classes she had instructed during three decades of teaching.

A fun sort of chaos was about to ensue, and only one person in the building didn’t know it.