We've got huge amounts of fluoride stored away in the body. That's why milk is a "problem" for people because it's probably pushing some fluoride out of bones. Similar thing I think could explain teenage acne. Once kids start producing all those hormones, fluoride that has been stored in hormonal glands starts getting released.it depends, it can be the gut as well, i dont drink fluoride water, and for 3 months ( when i was working) didint even cook in it, and still had acne, when i was carnivore i got 0, even if drinking tap fluoridated water, if i ate any slice of cheese or plant food i got acne a few hours latter ( always).
But i dont doubt it can be fluoride as well, i think the dosage they use to treat the water varies from country to country. it may be worth giving it a try.