I think most people are inherently shy. No one wants to think that someone else thinks badly of them or think that they are silly or stupid. Reversing your shyness could be as simple as practicing talking to people. You don't have to carry on a long conversation to practice talking to others. Start by offering a smile or two when passing in the hall or at lunch. Try to project a happy public profile by smiling more and being self confident. Offer a quick hello or how ya doin' with a smile/grin and then work up to a sentence or two accompanied by a pause in your walking. Things generally come a bit easier once you or someone else "breaks the ice".
Always remember that those kids you're talking to are probably (not always) just as shy as you are, they just fake being confident better. Don't compromise who you are so someone will like you better. Learning who you are (your values and inner sense of self) are just as important as making friends. And remember that you are in the discovery phase of your life. You are choosing who you want to be or who you want to be like, and who you don't want to imitate and so is everyone around you.
This is the same advice I give my own children and it applies, not just to young children, but adolescents and young adults. The worst thing that could possibly happen if you offer friendship to someone is that they decline the offer. No matter the outcome you will gain valuable experience speaking and interacting with others.
Another option available to you that will assist with overcoming shyness is joining clubs in your school. When you join a club or organization it groups you with others that have the same interests. It's much easier to make friends and talk to people you don't really know if you have something in common, or a shared experience, that you can both relate to. Remember the key to success is practice, practice and practice.
Best of luck overcoming your shyness!!
I was a very shy young person and I conquered the problem.