I'm as ignorant as the day is long about this stuff, but wouldn't you sneak a couple guys up to the classroom windows to play peek-a-boo with the help of observers and maybe some flash bangs? Maybe while you got a couple more guys busting down the inside door? They have radios to coordinate this stuff.
See the diagram here:
https://www.cbc.ca/news/world/texas-police-response-reaction-1.6469540This is what I've wondered from the beginning. They knew what room he was in. Why not use a window to smoke him out with a combo of flash bang and gas? In any case he would have killed himself, been disabled, or exited the room into the hall where his welcoming party waited.
