“ we are going to sit in the bleachers for the football game” is one way to use “in” but another way I personally use “in” would be telling someone directions to where I am such as, “I’m sitting in the bleachers. Can you see me?”
Vs
“Sit on the bleachers.” The word “on” can be more of a current action or demand, such as a mother telling a child to sit down on the bleachers. “On” is used more direct, I find.
However, using both either way is completely fine. You can use it interchangeably. This is just how I see people using it. :)