It depends on whether you're primary or secondary. If the latter, you're not normally expected to visit before applying for any post and should only do so if the ad specifically mentions it. With primary schools, I know it's the norm and some heads won't consider your application if you don't. As you can't reasonably take time out of school for this repeatedly, I'd suggest doing a great deal of internet-based research on any school before taking the time and trouble to visit. As for the time off - if you can't do it in your PPA, it'll have to after school.