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?
There might be a misunderstanding here. On the one hand, there is no way, ever, you'd say в стадионе unless it's in the abstract, e.g. не видеть необходимости в новом стадионе. Only in that non-locational, abstract usage. The logic of в автобусе doesn't apply here: even when really pressed to use an "in" word, you'd prefer внутри стадиона to в.
On the other hand, a specific stadium when mentioned by name can require в. In fact, I think most do: в «Лужниках» vs. на стадионе «Лужники».
Usage of в vs на is basically something you learn with the word itself, that is some words go with в, others go with на, and you just have to learn it.
The most broad distinction is that на is generally used with open-space objects (like стадион in your example), while в is used with enclosures.
However, this is very vague and exceptions are so numerous that it's just better to say the preposition is something you learn with the word.
With some words the usage of the preposition varies with the speech register (prison argots use на тюрьме, на зоне, на больнице, while normative Russian uses в); regional and municipal preferences (в Украине, на Лобне, на Сходне); awareness of tradition and etymology (на ФГУП, на Физтехе) etc.
So, in your case, на стадионе is correct and *в стадионе is not, same as (я видёл её) в автобусе is correct and *(я видел её) на автобусе is not, just because there very words require these very prepositions.
