There are two valid arguments against putting cops in schools.
First, it takes cops away from the rest of the community, so while schools are more safe, everywhere else is more dangerous. If we hire more cops, where will the money come from?
Second, it teaches the wrong lesson to the kids. To live constantly under the watchful eye of an armed authority, taking no responsibility for your own safety other than to run to that guardian for protection, will have a terrible moral cost. Placing uniformed guards in schools without permitting the people being guarded to handle their own problems just teaches helplessness.