How do I start a conversation with her ???

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i m in 12th standard & there comes a new girl in my bus whom i really like, but the problem is she sits/stands far in front with other girls and i sit/stand right at the back with other boys. i think she likes likes me because whenever we have an eye contact she smiles & visa-versa . The actual problem is, there is another girl with which she sits who had said that she had a crush on me but i refused her, and now she hates me and we haven't talked since then ,so i wonder what she might have told this girl whom i like. i cannot just startup a conversation with this new girl in front of this other girl who had a crush on me, in other words she acts as a barrier between the 2 of us, & they both get off at the same bus stop. they r good friends now i have had many girlfriends but this is completely different. the rest of the girl gang won't let us talk alone i know that, it has happened before with other boys also. please help me how should i start a conversation with her so that she leaves alone the the other girl who had a crush on me earlier.