@SilverChaika some additional things I found:
3: she's the one that plays family with the kids in the sand pit, and later plays as doctor Black Jack with the fake scar. maybe she's a game master. or just the sand pit master...
edit: she's also the first one to enter the robot's candy store. it might be to further emphasize her child-like nature.
5: she sits right in front of the teacher podium and often the first to answer them. eventually, she's also the one that usually starts an activity, like making a club. kind of a "class-rep" character, or maybe the most main character-ish of them all. incidentally, she's also the one who initiated conversations with the other clone girl group, and the one that showed her eye number on the other group was also a no.5.
7: often the solution giver. whether or not her solutions work is another thing though.
8: I can't really tell if she has anything unique. but, if we think of this another way, if all the things that we mentioned makes the other unique, she automatically becomes unique as the last remaining one that retains the, more or less, original personality.
edit: incidentally, she's the one that stole the female teacher's clothes and became teacher for one day.
16: I noticed that they might had started treating her badly after she got brought in by the police while wearing the raven head. the next chapter was the escalator where the other girls refused to high-five only her. considering the number girls reaction during the shop alarm skit, it might be that these girls hate criminals (that actually breaks law and get caught by authorities, unlike no.2 who's mostly more of a naughty kid who breaks vase). and they started treating no.16 badly because they arbitrarily decided that she's a criminal.
side note: 2nd, 12th, and 15th are the ones that, according to no.1 and 7, "Don't read books at all so they must be stupid". the three stooges? the bubble gum skit features 2, 12, and 15 where 2 as the "delinquent" girl can eat gums, but 12 and 15 always swallow them.