Not for nothing, but if someone comes into a classroom with a firearm and starts shooting, the first thing everyone is going to do is panic, with most people trying to run away. Unless you are sitting right in front of the door the perpetrator walks in at, it's going to be tough to employ any type of defensive mechanism. You would most likely end up getting trampled by the other students in the classroom....especially in a large lecture hall setting.

Not to mention, you're already at a tactical disadvantage, as they are most likely better armed, they've already got their weapon in their hands, and they may even have the element of surprise on their side.

With this being the case, first thing I would do, if I was in the kind of situation, is throw my backpack over my shoulders to help cover my upper body and start running towards the closest viable exit. The idea being, between the chaos of everyone running away and having my book filled backpack covering my vitals, I would lessen my chance of receiving a fatal injury as much as possible (short of going to class wearing a ballistic vest.)

Even if I had a firearm on me, you can bet your butt I would be running unless I was absolutely cornered or found myself unable to move and had to fight back. Sometimes it's best to know when to retreat and regroup.