You're generalizing, and those are different situations.
In however many years, that age difference wouldn't be a huge difference, but it is now. Her dating you would be borderline illegal. If you asked her out (regardless of how it turned uot) if someone heard about it, It would draw a connection between you, and people would suspect wrong doing on her part with your marks, pedo, etc. Even if you waited until after school, people can still make connections, regardless of whether they are false or not.
In regards to (getting to know someone better) in a bar setting - that is also different. One, you're forgetting the consequences both of you would encounter - which have been discussed. Two, that is a setting where people usually go to meet people.
Would you really expect that if you two madly declared your love for each other, that everyone will be fine with that? Think man. Men dating older women is okay - to a point. But Circumstances DO play a part in things. Sorry dude, thems the breaks. I don't know how many more times this board has to tell you this is a bad idea.
I don't know how else to say things to you?