Getting permission for primary schools is the right thing to do. I classify high schools though as fair game, especially if the sporting ovals are used by the council for summer sport, etc.
Our high school even hosts an All Ford Day on one weekend per year - kids and adults take their dogs for a run - so lots of both authorised and unauthorised activity. I tend to go when it is empty - maybe even a little unpleasant weather wise - and I never get approached at all.
It's funny, but I read a lot of apprehension regarding detecting in various places, and yet I have never been asked to leave anywhere I have detected - curiosity occasionally but never asked to go. So perhaps you say to yourself until I am asked to leave let's push the envelope and just get out there and go for it! I would think detecting in broad daylight is less conspicuous then lurking around after dark - less intimidating and easier to see the harmless activity from a distance.