You’re right. With 3 officers, I’m sure one of them could have hit his leg from behind some cover.
Clearing the place out? Shooting him in the leg? Are you utterly stupid? If they suspect that it is a firearm in a school, you do NOT have hundreds of kids filing past a loaded firearm. You also do NOT shoot a gunman in the leg, and allow him to start killing people while he remains there, in pain and now under the most intense fight-or-flight response.