So, more details are emerging, and it's looking like the school lacked security or a SRO, and the cops failed to aggressively engage the shooter and actually stop him, until a team of border patrol agents arrived and actually killed him.
A lot of details are unclear, but the most plausible sounding version of events is the shooter entered the school unopposed, shoot a bunch of kids in a classroom and barricaded himself inside, then drove off the first wave of police responders. The shooter then stayed put for the rest of the incident, until the border patrol finally arrived and put him down.
It's doesn't appear that the police's failure gave him more time to keep shooting, but it's very possibly that the delay in getting medical attention to the wounded lead to additional deaths.
That said, this is based on putting thought fragments of a bunch of media reports from varying sources, so I can't say for sure this is what happened. It could be just as accurate as the "shooter was engaged by cops going in and was just unstoppablefor some reason, good guys with guns don't work hur dur" narrative that was getting pushed out yesterday.