I can only share what I've done and tell you I think it has worked well. I have two daughters ages 17 & 19.
Both of them told me when they were 16 they were going to work. I made the rule that they could only work weekends, not after school, because they have 50 more years to work... and only 2 or 3 to be a kid. So, both went to work at McD's on the weekends. McD's accomodated their school and activity schedules. Both have 2 years there on their resumes... which also looks very good for their age.
Both were accepted to a prestigious private college with a signifcant amount of scholarships and financial aild. But, I still have a chunk to pay. My ex, does not help at all.
Because I have to pay an amount that stretches my budget, my oldest works at a local church in the nursery during Sunday and Wednesday services. This provides her spending money... that's on her. I had hoped to be able to provide her a small allowance monthly ($100), but it's not been in the budget so she has been fine with her part time job. She does not have a car at school, it's not necessary where she is. That's lucky, I know.
My youngest also plans to work part-time when she goes to school there and is already scoping out the job market.
They like the independence they receive from working. They like having their own money to choose to spend however they want.
Working and going to school is very difficult. I know. I did it. It took me longer and really cost me more than it should have. There were times when I couldn't afford books and other expenses and eat too... I chose to eat. So, I want to help my daughters succeed, but I want them to "own it". I think we share the burden well. Both girls will pay a portion of their school fees through a work-study program (not federal).
As for prom, I would say we sort of shared the expenses. The dress was provided, they bought their accessories. I paid for senior pictures, etc. If they wanted extra or anything different, they paid.
My daughters have paid their own auto insurance and before she totaled it, my oldest was purchasing the family vehicle she was driving from me with monthly payments (I set them aside and used the money to help move her into her dorm). Both have been given a gas allowance (based on the amount of milage required for school and after school activities) and anything over that they must pay (travel to work, running around with friends, etc.)
My oldest backpacked around Europe after graduation last year with her dad. I am not wealthy. My ex, is not wealthy. He saved his money (or used his cc, don't know - don't care). My daughter saved for over a year to be able to make the trip. She put money aside from her McD's earnings and asked for cash rather than gifts for Bdays and Christmas. She reached her goal. I paid nothing towards this trip. She bought her own backpack and supplies.
My youngest will travel to Germany in a brief exchange program after graduating this year. She, too, is earning and saving the money to achieve this... she's almost there! Her German guest is here now and she has been using her money from McD's to pay for entertainment for them both.
So, a very long post to basically show that I worked for a balance. They are, after all, still kids. But, we are raising them to be adults. For me, that means, having them take certain steps to more responsibility, etc... with a safety net and support.
I think one can help their young adult be responsible without just tossing them out into the cold, cruel world to make a plethora of (costly) mistakes. And, I think one can provide support without spoiling a young adult. Support does not mean total dependence.
I'd say, do what's in your heart... deep down, you know what's fair and right. Between the two girls (great that they both got such wonderful scholarships!) keep things equal... if no other way, use monetary measure. That way, you can know you assisted them equally as best you could.
Congrats, good luck!