11:30am: First 911 call.
11:31am: Officer arrives at school.
One minute police response time, by your metric. ~80 minutes later, the ~400 very well armed cops on scene managed to finally go in.
A cop outside is not obligated or expected to put his own life at risk. He's expected to show up for work tomorrow.
I do not agree with the second at all. Law Enforcement are both obligated and expected to put their own life at risk protecting the public. That said they are also expected to show up for work tomorrow! It’s a sad day when they don’t.
A cop can be at the scene of you getting actively murdered and do nothing about it, and legally the cop is ok.
Morally, you could definitely argue they're not okay.
1981 Warren v. District of Columbia
2005 Castle Rock v. Gonzales