Kozushima High School is the school that Hodaka Morishima attended during his teen years.
Little of the school is shown in the movie, but it's seen to be an average-looking, one storey high school that extends a far length across its foundations, curving in an "L" shape towards its left. It has a large yard outside of the main building, which appears to be empty.
Not much information on the school is given, but Hodaka notes that he that felt it was "chaining him down"; it may have been a contributing factor to his decision to run away to Tokyo. The uniforms given to students appear to be rather typical for Japanese schools, as the boys wear black gakuran jackets and the girls wear blue and white seifuku uniforms with plain blue pleated skirts. Also according to Hodaka, there were about thirty students in the school by 2024.
Hodaka was enrolled in the school at some point during his teenage years.
After being sent back home following his Tokyo capture, Hodaka begins attending Kozushima once again and stays there until he graduates. During the three year gap, his life in the school improves somewhat significantly: his grades become better, he gains more friends and he becomes a social butterfly with adults. His graduation ceremony takes place in the school's main hall. After the ceremony, two female students approach him and nervously ask him whether he'd been arrested before.