Maybe the students wore sweaters, until just before picture time.
We don't know the entire story.
That's what I am assuming, it was either that or the school is pretty lenient on the dress code, but didn't want the year book to reflect kids violating the dress code.
Or, considering they showed two different girls wearing the same tops and only one was edited, perhaps some of the girls push the limits on the dress code and someone made sure their photos got a looking over.
That seems like a plausible theory.
Or, various people checked over all the photos and what one passed, a different person did not allow to pass.
Years ago, I would have thought dress codes were ridiculous but now that I am older and read that kids who all had to wear the same outfits, seem to do a little bit better in school.
The thing is, almost from birth, females are in competition with each other, all the down to the color of their shoes.
Remove that from school and its one less bullying thing and/or one less thing for girls to have to worry about.
No jealousy because so and so has the cutest skirt on that day and its the most important subject of the day for all the girls,
or all kids making fun of other kids who cant afford name brand outfits.
Makes perfect sense, I know that's kind of off topic to this schools dress code though.