I read on one of the other threads that the SRO was shot (but not killed). So that may be why he failed to engage.
More information will hopefully be forthcoming, as there is some confusion on this point:
“The Texas Department of Public Safety confirmed a school police officer exchanged gunfire with the suspected shooter who unleashed fatal gunfire at Robb Elementary in Uvalde.
“ - see https://www.foxnews.com/us/texas-school-shooting-sro-officer-gunfire-shooter
Other questions that will hopefully be answered:
1) How did the shooter gain access to the building (via locked door, unlocked door, etc.)?
2) Did the school make use of any materials at entry control points such as thin film laminates, bullet resistant glass, etc?