17 here. "he scares me 'coz he makes me feel dumb" is just another way to tell you (through your friend): "Your elitism makes me feel inferior. I am not at your level. I am afraid to even approach you because I feel you are making fun of me.". The "he's too serious about things" part may mean "I am different from you in the way I like some things and you don't. I am too proud and too scared to change because I'm afraid you will look down on me. I just can't fight the impression of you constantly thinking about my inferiority. If I admit that I like you that will mean I surrendered to you. And I think you don't even like me. Maybe I'm too stupid and not worthy compared to you, but I still have some dignity. The last thing I want is to open up and feel vulnerable in front of you"
Learn to read what people are saying. If she just said all this to you, life would be a lot easier than it is. But she won't so use your intelligence and get in her shoes once in a while. She obviously throws the ball to you because she is acting defensive. She doesn't have the guts to tell you all this in your face so you're not the only one who's afraid of confrontation. From all you said so far, if she wasn't interested in you she would cut it short in the beginning and would never even ask for your help. I believe you're not the only brilliant person in the whole school. She clearly wants to initiate some dialog. I am repetitive here, but be aware that both of you share a lot of the same fears. There's one major distinction between guys and girls: guys are expected to surrender first. So, keep that in mind.
And even if it turns out none of this is true and she doesn't like you, it was about time to either start it or move on. You two spent years in that status quo - it's just not healthy...
And about you not having guts to talk to her.. It's just few seconds in reality... You spill the words and from that point on the ball is in her court so she'll have to deal with that. But make sure to say what >>19 told you: I like you so I don't want you to think of me that way.
Your problem is that you pay too much attention of what people think of you. Keep in mind that they're defensive. By thinking about you, they try to escape from their own realities and issues... Will they change anything by simply talking about you?? Yep, they will change whatever you let them to... Focus some of that energy you spend on those irrelevant things back to this girl... And you need to ask yourself if you really do like everything about her. Maybe you're not OK with her drinking, being more extrovert than you and so on. It's all things that stand in your way. Same goes for her. Sounds cheesy but fate is building a bridge of chance for your love. You gotta accept some things the way they are.
Oooh another tl;Dr :) Make a move and let us know how it went.