Start with the rooms he's not in; evacuate the teachers and children to a safer area while a sniper sets up outside the classroom.
Heroine Angeli Rose Gomez drove 40 miles to the school, got handcuffed and then released, jumped a fence, ran into the school, grabbed her two children and got them out.
She proved she didn't need specialized training, a firearm, body armor, a ballistic shield, "intel", nor a tacti-clown team. The other law enforcement who got their own children out proved the same.
https://freerepublic.com/focus/f-news/4066632/posts
Woman armed with a pistol shoots and kills a shooter who had an AR-15 and was shooting into a crowd.