Fixed it for you

Officer's gun accidentally fires in San Antonio Middle School

Ummm. Something's not quite right with that. Let's try this.

Officer accidentally fires gun in San Antonio Middle School

Better but still not quite accurate.

Officer negligently fires gun in San Antonio Middle School

I think we're almost there.

Officer negligently fires gun in San Antonio Middle School because the nitwit had his finger on the trigger

That's damn near perfect but it'd never make the story's lede.

H/T to The Duck via Uncle.