School is a very easy topic to do. Depending on what kind of art you are good with you can decide what would best. A girl or boys' only school would be good if you specialize with a certain gender and mythical creature or furry schools also make good comics. If you specialize with fight scenes or super powers you could have a battle or superhero/villain school.
Daily life comics are good for artists who have a hard time repeating character designs however a superhero or magic comic is good for artists who don't like to repeatedly making up outfits. Read some mangas or comics and decide what kind of art style and storyline you'd like your storyline to have.