COLD SPRING, Minn. (AP) -

A Minnesota school district where two students were killed in a 2003 shooting has unveiled a new device aimed at adding another layer of safety against future attacks: bulletproof whiteboards.

The Rocori School District has acquired nearly 200 of the whiteboards. The manufacturer says they're stronger than police-issue bulletproof vests.

The idea behind the whiteboards is they can be used for lessons one minute, then quickly turned into shields by teachers and students in an emergency.

The maker, Hardwire LLC, says the whiteboards are already in schools in North Dakota and Maryland.

One security expert questioned how effective they would be. But Rocori officials say they are just a supplement to other security measures.