What you mention (angles, oof, composition rules, etc.) aren't really themes, they're just tools to achieve a goal that you've set yourself before.
If I were you, I'd try to find a guiding topic, a theme, a common idea. What do you find interesting about the school, visually? The architecture of the school? The harsh flourescent light and the colours? The eery weirdness of an empty school building (I always find empty schools creepy, e.g.)? What about the traces your fellow students and the teachers leave behind? Chewing gum under the seats, confessions of love carved into school desks, traces of the last lesson on the whiteboard, etc. The possibilities are endless.
What you mention (angles, oof, composition rules, etc.) aren't really themes, they're just tools to achieve a goal that you've set yourself before.
If I were you, I'd try to find a guiding topic, a theme, a common idea. What do you find interesting about the school, visually? The architecture of the school? The harsh flourescent light and the colours? The eery weirdness of an empty school building (I always find empty schools creepy, e.g.)? What about the traces your fellow students and the teachers leave behind? Chewing gum under the seats, confessions of love carved into school desks, traces of the last lesson on the whiteboard, etc. The possibilities are endless.