Two High Schools, two different behaviours

I have two games involving teens going to high school. One school I created myself (by modifying a large residential lot) and one I got off the Gallery. In the latter as soon as I arrive at school, students are milling around the building doing various things until it's time to head for the classroom. But in the former (the one I made myself) they just stand in a group outside until it's time to start the first class. I do use some mods, but as I use the same ones for each game, they shouldn't make any difference. Is it do with where I placed things? For example, I found the lockers unsightly, so I put them in the attic - still perfectly accessible up a couple of flights of stairs but just not in your face. Any ideas?