What are some good jobs where you mainly work with 18-21 year old girls? I'd like to have fun flirting while working at shit low paying jobs in college.

Tutor.

Professor.

male prostitute

This is better than the high school gym teacher suggested in the OP's original thread and much more legal.

High school gym teacher.

To get even more specific - professor in the humanities or visual arts. Totally the sort of impressionable ditzy girls you want.

Hard Sciences not so much and not enough girls - as for law... you'd be crazy.


EDIT:
and I should clarify when I talk about visual arts it doesn't even mean flirting.
a chick I know said to me "yeah there aren't many rules here. Like it isn't unusual for students to hook up with staff"

Office

Or my best job was surf shop

lol.

To OP:

Coffee shops are usually full of young girls, both staff and clientele.

Lifeguarding, I was 16 and somehow able to pull an engaged and smoking hot 22 year-old. A benefit to this would be requirements dictate you wear sunglasses constantly while on the job, so there are no issues with coming to work drunk/high as long as you don't fuck up.

I was a stocker(pretty manly job since I had to lift boxes of steel pots and pans every day) at an upscale kitchen supply store. The ladies working there LOVED it, I got thank you's and compliments constantly for helping them move heavy shit.

None of that compares to camp counselor though, you tend to meet very easygoing and chill girls working with kids because the high strung ones can't handle it. Usually stoners, and always hot.