If i am a teacher with a CCW and "a good guy with a gun" small enough to conceal

[loose wheel]

12. A classroom is a tight small space

Inside 20 meters, any decent caliber handgun is just as useful as any "assault" rifle. Proabably a little more so to be honest, the handgun is a little more maneuverable and easy to fire. The weapon at the link below is a good place to start looking. It's a 9 mm semi-auto with a barrel length of just 6.1" and a weight of 19oz. It's easily concealable on your person, and anybody that spends an hour or so at the range every month practicing can be proficient enough with it.

It could even be loaded with flangible rounds so it won't penetrate thin walls while remaining a threat to any assailant that happened along.

http://www.smith-wesson.com/webapp/wcs/stores/servlet/Product4_750001_750051_809560_-1_780153_757781_757781_ProductDisplayErrorView_Y