I know what it's like to get picked on by kids younger than yourself. You can't do anything physically because you can get done, so the next second best thing is mind games. Like others have suggested, smile back, say hello, remain calm, sit down for your journey, if they start talking to you on the journey, talk back to whatever they say. It will confuse the hell out of them. When one of them next gets dared to talk to you and says hey sis, say hey bro how's it going pal? I can guarantee they'll be like what the hell and feel like an idiot in front of all their friends. I'm not saying what they're doing is right, because I hate bullying, but showing them you're annoyed is giving them what they want. Joke with them, even if it's not funny. I had some girls call me fat, so I said yeah and what? They didn't know what to say so they moved on to my hair, my clothes, my skin, everything they could possibly think of I agreed with and laughed along with them. Even though it was laughing at myself, it made them realise they're not the big people they think they are. Play along with their jokes. They're pathetic 11 year olds and the less you show you're annoyed and the more you show it doesn't bother you, the faster they'll leave you alone.
Joey