I think I missed this point in my first year of study. Is it correct to say:

"я был в стадионе на футболе"

I learned that на goes with words like стадионе, but today one of my roommates mentioned that we can use "в" here. Why is that? His teacher said it's proper to say "в".

I'm aware that it's correct to say: "Я ее видел в автобусе". Is the above similar case?