Caution: Spoilers have been unmarked. View at your own risk.
After the movie, the Cuties' parents were arrested off-screen for letting their daughters participate in a dance group sexualizing minors.
It seems likely though that Amy's mother was at least let off the hook since she had been struggling with domestic marriage issues and was shown to be a vulnerable person: not that that's an excuse, but she did her best to discipline her daughter, however her husband's wedding was so important that she just couldn't balance it with her duties as a mother.- After that, the Cuties will be given therapy sessions.
The Sweety Swags are responsible for the Cuties' Freudian Excuse.
The Cuties could have initially been sweet little girls who performed ballet or the likes, living up to their innocent name, but once they begun rivalry with the Sweety Swags (either through bullying or being outmatched in a dance contest), the Cuties got jealous of the Swags' superiority in clothing and dance moves, wanting to appear Hotter and Sexier. Thus began the scantily clothed and twerking-obsessed dance quad. The Sweety Swags experienced a Heel–Face Turn following Amy's departure from the Cuties.
Tying into the above WMG regarding their parents' arrest, the Swags may have had a Jerkass Realization following the news of the girls' parents being put in jail and the girls' subsequent loss, with both the Cuties and the Sweety Swags realizing that they didn't need to be people they're not in order to fit in with the crowd (much like Amy did at the end of the movie), officially starting a new friendship. The school they go to has No Dress Code.
What else would explain the staff's ignorance to the girls wearing skimpy outfits?