Also people do stuff like wearing cute socks with pictures on it instead of white ones, wearing white shirts of a pattern different from the school one, wearing cool sneakers, etc. etc. Also people put their hair up in ways you aren't supposed to. They wear bright hair ribbons, and put all sorts of keychains on their school bags. The more daring girls might dye their hair, or perm it. One girl in my high school class was forced to go to the barber and have her hair dyed back black after dying it reddish-brown.
Some people also experiment with cosmetics.
In Japan you don't get to decide whether to wear the summer uniform or winter one just by if you are cold or not. Everyone in Japan changes from winter to summer uniform or vice versa. If you just changed to winter uniform and then it gets really hot in late October for some reason, too bad. It was so rigid. Skirts had to be exact lengths.