No expensive gimmicky backpack to buy, just bring your own, stick it in and off you go.  Fits in any standard size backpacks.
NIJ IIIA, 0101.06 standard
One of largest coverage on the market, approx 12" x 15.5"
The panel protects you against: 22 (all), 25, 32, 38, 380, 9, 40, 45, 357sig, 357mag, 10, 44 special, 44mag, all +P rounds, 12g shotgun slug
Exclusive dual layer build in high impact trauma boards to minimize the blunt trauma injuries, one of the kind on the market
Outer shell is made of waterproof synthetic fabric with heat sealed edges
Ultra thin: approx 1/4" 
Ultra light: approx 1lb 12oz  
Airport, school and government building friendly.