On the video at the link you can see that the administrators and teachers/ maybe parents were watching via live feed. They interviewed the students and according to the interview, it sounded like they knew it was a drill.
The SWAT equipment came from a federal grant. Militarization of local police is a federal program.
I would imagine that anti-terror folks may be concerned about Beslan-style attacks here in the USA.
I hope that the kids knew in advance. I would have peed my pants like a champion.