The building was placed on lockdown immediately. A state police canine unit was brought in to search the building. But by late morning, police determined it was safe to evacuate the building floor by floor and send the students home around 11:15 a.m., said district spokeswoman Kimberly Brehm.

Hmmm ... the students were kept in the school where the first device was detonated until the rest of the school was searched ... and THEN they were sent home.