So THIS girl is answering. I think it is wonderful that you are growing in your hashkafa. That being said, it is incredibly rude if someone says hello (and I do not care HOW frum you are, this holds true across the board), if you do not respond b’sholom yourself. We learn that it is incumbent on us to be MAKDIM someone b’sholom. That does not mean you have to stand around and chat if you feel at this point in your life that this is wrong for you. But you do have to be a mensch. The young men with whom you are acquainted deserve respect also, even if they do not share your hashkafa. If and when they ask you about this at some point, you can explain to them what you did to us.
But on the spot, you don’t have to give long-winded explanations to people. Give a friendly hi, how are you and move on. Frum does not equal cutting people off when you have known them all your life. That is a big mistake that some people make, in my opinion. No one has to have a conversation with someone who for whatever reason makes them feel uncomfortable, even if it is only because they are of the opposite gender. But no matter who you are, you always have to be a mensch, and that means you don’t ignore people whom you know who come over to say hello. As I said, you smile, keep the conversation very brief and move on. The Kiddush Hashem is in living a frum life, despite the challenges presented by others. Just my opinion.