I think it would be fine. Here for the first few lessons Jean's are fine ad long as you have appropriate head and footgear. Best is to call the instructor.
My son refused to wear breeches for years but instead worn a polo(the sport) jean that was soft and super stretchy but otherwise fit and looked like jeans. They were a light tan. He would wrap the lower leg and put the half chaps over them. Made them look like breeches without the overly close fit he found uncomfortable.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.