Learn English – Using a comma before or after which

Your existing sentence, with the comma after which, is fine.

In this case you're using commas to indicate an aside, or a parenthetical phrase - something that is not essential to the understanding of the sentence. The sentence:

Our party will be held in the cafeteria which is located inside the Students' Union.

...would be fine and comprehensible by itself. The added phrase:

for those who are unaware

...is an aside, a parenthetical phrase which adds some context and depth, but is not essential for understanding.

Stylistically, it may be helpful to ALSO have a comma before the which, to separate the two clauses, but without the parenthetical:

Our party will be held in the cafeteria, which is located inside the Students' Union.

I'd imagine it's this usage which is tripping you up. However, the comma before the which in this case is doing a totally different job. It's separating the declaration of the party's site from the declaration of that site's location. Both parts would make reasonable sentences:

Our party will be held in the cafeteria.

The cafeteria is located inside the Student's union.

Combining them into one is absolutely fine, but it is helpful to use a comma to separate the two clauses.

You COULD have commas both before and after:

Our party will be held in the cafeteria, which, for those of you who are unaware, is located inside the Students' Union.

That would be correct, but feels awkward. You CANNOT have just a comma before:

Our party will be held in the cafeteria, which for those of you who are unaware, is located inside the Students' Union.

This does not work.

An alternative might be to use a different mark to indicate the aside. A dash or actual parenthesis could work there:

Our party will be held in the cafeteria, which - for those of you who are unaware - is located inside the Students' Union.

This avoids the awkward double comma, while still picking out the parenthetical phrase.