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-1.6469540

Politics is War by Other Means