In fairness too, "formal" for a bunch of kids out in the boonies is not the same as formal in London.
My mom and I made my prom dress. I think it cost some time and about $30. I borrowed jewelry from my aunt and I got a new pair of shoes for about $15 that were then, my "Sunday" shoes.
Anybody who couldn't afford to buy a dress had use of the home economics sewing machines to make their formal wear if they didn't have a machine at home. There's always an older woman willing to teach you too, just to keep the skill alive.
I think the best dressed was a gay kid who designed and made his own suit. He was looking pretty sharp. Miss Dot helped him out.