Suggestion? Don't. Just don't. It is a felony to bring a knife on a school campus in lots of places, and that applies to teachers too. I was a teacher for almost 8 years, and the biggest thing I brought was the dinky little SAK with a 1 inch blade because it would have been my career to have more if someone wanted to make a stink. No knife is going to level the playing field for an active shooter, period, that's why the active shooter protocol is 1)flee 2)shelter in place 3)fight back. You would be better served by an effective door wedge that would keep someone from opening your door even with a key. If you want a "weapon," keep a hammer in small tool kit in your classroom, along with a good size screwdriver. If you want something you can carry, go visible, not hidden. A Vietnam vet I had for English class when I was a lad was always leaning on a yardstick in class; it was his prop. He pointed to stuff, leaned on it a lot, and heaven help you if you snoozed in class, the flat side of that thing on a desk was like a Gabriel's horn calling for battle. Years later, I realized that his fidget toy would have made a great weapon, one quick movement and two stabby bits. His hardware store ruler broke in class one day, and some of his students made him a really beefy one and they all signed it--he had it years later. Storytelling aside, in all seriousness, keep your blades at home.