People think I'm a dumb blond.?

I know I'm probably going to get a lot of people mocking me for asking this question, but I honestly don't know what to do. The other day I found out my ex boyfriend called me an airhead behind my back. My boyfriend's friend always makes fun of my voice and stuff. Sometimes if I'm at school and I am talking to my friend or something, people will start imitating me. I am sort of blond, and I know some people think I'm a dumb blond, plus I have a California Girl voice, which irritates a lot of people. It never really bothered me until I found out people were calling me an airhead and stuff. I don't want to sound conceited but I think I'm pretty smart. In class, people always tell me I'm smart and not in a sarcastic manner. People always want to work with me on projects and group stuff because I'm pretty smart. Teachers always tell me I'm smart. Sometimes I don't get the best grades because I don't put a lot of effort into my work, but when I do, I do really well. I don't know how to make people stop thinking I'm a dumb blond. How can I start to change the way I talk? I know my voice gives off the wrong impression. I don't know what to do, please help.