Japanese schools have many more ceremonies and formal functions than western schools do. For graduation, staff picture day, and the new student entrance ceremony, a suit jacket is a must. Term opening and closing ceremonies differ by school and season. During entrance exams and PTA days, most of your Japanese co-workers will be wearing suits, as well.
Guys may want to have a couple of suit jackets on hand, but for a lot of girls it is far more lax most of the time. Often just having one good suit jacket to pair with a nice dress or skirt on important occasions is enough.
The formal work attire that you wore or will wear at Tokyo Orientation is the level of professional formality required at school ceremonies and other formal events. Many Japanese teachers only have one or two suits, especially female teachers, so it’s not uncommon to wear the same outfit to every formal event.