What can I do about my mates cos some of them are being really out of line when they talk about my girlfriend?

I'm sorry you are being bullied by your fellow pupils at school. (Notice I don't call them your friends.) The best way to deal with bullies is to ignore them and not let them see that they have upset you. Keep your dignity. Enjoy your online friendship with Gina; getting to know and understand people from different cultures is the way towards peace. In future, think carefully before confiding in other people until you really know them (your mate obviously isn't a mate at all, I suspect he is jealous of your maturity). It will get easier to recognise who potential friends are as you get older. Stick to your principles and feel sorry for these people who are teasing you.