When (blue)-jeans becamed popular in U.S.A?

[Airedale]

The old man griped about not being allowed to wear jeans in highscool, then the year after he graduated, '68, his high school finally allowed their students to wear them.

He was a farm boy so he wore jeans when not at school. While jeans sure arent considered formal wear, it's hard for me to imagine an era where you were forbidden to wear them at nice restaurants or high school.