Our company helps harden targets with two primary window film products. Both of these products can be applied to most existing glass without any retrofitting of the building. Once installed, very few people will even notice they are on the glass. They are applied without any chemicals and with very little downtime. A brief description of each:
Riot Control - This product is used to delay/deter entry. While it will NOT stop bullets, it is a very impressive product. Even after being shot, it is strong enough to cause a significant delay to entry of the building.
Bullet Resistant - This product has UL and NIJ third party testing certifications for bullet resistance down to 1/4" thick glass. It is one of the most impressive products I have ever witnessed.
Kevlar like Paneling - for under desks, walls etc.