It may have been an error to ask for the next Friday after her telling you that Friday's are reserved for her bf...If the opportunity comes up again, go for another day.
You have put yourself out there, now its up to her to decide whether to respond. It sounds like things are going strong with the bf at the moment if she's reserving days for him.
I wouldn't ask again unless something drastic changes, like her telling you that she has nothing to do on Fridays any more. You let it be known in a nice way you want to see her outside of class, she declined.
Don't remind her...if she needs reminding, she isn't interested.
Remember, she's in a relationship. If she started seeing other guys while still having a bf, she wouldn't have a very good character.
Don't make anymore 'get together' moves until you know she has gotten out of her relationship. Keep the conversations fun, so that she looks forward to seeing you. At this point, that's all you can do.
And if by any chance she gets back to you and says "I can get together on some other day..." make sure you don't refer to it as a date. Just keep it light, make her laugh. If she decides to "test the waters" you don't want to make her feel defensive or guilty about hanging out with you.
At least you know she isn't a cheater...