I think the intent is that anyone employed as a LEO,
whether on duty or not,
whether in uniform or not,
whether on official business at the school or not,
will be allowed the special privilege of keeping & bearing arms in the building & on the grounds of a school.
(Just like licensees are allowed to do across the country, with no problems, so why limit it to LEO?)
It apparently even overrides the property rights of a private school to exclude certain citizens whom they deem to be undesirable.
[ETA: and I don't see a restriction for LEO from within the state, so out-of-state residents would once again have more rights than WI residents, within WI.]
Correct me if I'm misremembering, but the law already has an exemption for on-duty LEO.
So this is one more instance of giving extra privileges to people based on their employment.
Extra privileges while doing their job, fine. Sometimes they're needed.
But someone who's off the clock is (supposed to be) just like everyone else... until there's an ND & the Chief says his employee shouldn't be treated like everyone else, or until there's a DUI & the officer isn't treated like any other citizen. (Both Milwaukee OD officers, but I'm sure similar things have happened across the state.)