Why doesn't Marika wear same uniform as the others?
In this case, Marika is probably wearing her old school's uniform. As a transfer student, there's a grace period before you get provided the one the school designates (they are typically purchased).

This is also the case of the Transfer Student Uniforms trope:


  New Transfer Students in places where school uniforms are a cultural default setting often wear their old ones until the school can provide them with a new one. In fiction, this shows the newcomer or outsider. When they get the current school uniform, this indicates they've been assimilated. If the student is meant to be a Fish out of Water, they will keep their old uniform throughout the series. In Japanese media, even rebels don't choose to abandon uniforms entirely; in American media they will do this even if the new school doesn't have uniforms. Compare the Non-Uniform Uniform.