Police officer shoots and kills teen. Doesn't report it.

We have organizations who want to improve the teaching pool by judging teachers on how the students perform on tests.
I don’t necessarily agree with that, but its a national thing.
Why can’t we have national standards for hiring & training our police forces?
I realize that most officers are not the problem, but wouldn’t the good ones want to get rid of the ones who aren’t?
When we see police making bad decisions under circumstances that shouldnt be that difficult, I wonder what would happen if they were in a * really * stressful situation?