I'm a girl and I'm bisexual but everyone starts to tease me and say I'm a lesbian what am I supposed to respond to them?

First of all, don’t give into their hate. You are who you are and you know who you are. The hate they give you is NOT acceptable by any means. Don’t let them push you around. You are valid, and being bisexual does NOT change that.

Try to not let them get to you too much, people who feel like teasing you for only half your sexuality don't really deserve your time. Be confident in who you are and you can always say something witty in return to the teasing comments, or saying nothing at all. Just don't let them get the better of you because you are amazing, no matter what.

If they do that because they honestly don't understand what being bisexual actually means, that could be because bisexuality, unfortunately, is still not a widely known reality, and people often misjudge what they don't know. But you can explain to them, patently and with an open heart, what it really means to be bi. If they're doing that just to tease you, you can always tease them back! Not in order to deliberately mock them: you can do that with a smile on your face, so they'll think all this teasing doesn't affect you and with time it should discourage them.

It is your life and your sexuality. If individuals feel they have the right to comment on that, hey are incorrect. However, if you would like to explain to them that you are bisexual then you can say just that. Explain to them your feelings towards your sexuality, and educate them because perhaps they are teasing because they are not sure how to respond. It is not appropriate for them to tease you, but by you explaining then perhaps you will feel better by providing them some knowledge on the subject. Do what you feel comfortable doing, that is the most important thing.