Say, "yes, that's right. I'm an adult, and you are children. Now, do xyz." Girls that age hate the implication of being "immature" or childish.
This is what the '***** slap' was invented for!
I'd laugh, and say "You got that right 'little girl(s)!"
Don't LET them see it BOTHERS you!
Smile and MEAN it!
Besides you are a young woman.
I know I didn't like hearing myself being addressed that way, (or "Ma'am).
I just smile, and give them a look like 'I know something you don't', (makes my smile bigger, and more mischevious).
I'd take it as a compliment.Nothing wrong with being a "woman". ;)
Change your attire possibly
Since you are 19 and hanging around school of junior high might be an issue too dont you think
Call them "******".