Just be yourself. Girls will understand how you act and behave all the time so it's not use changing habits and having to sustain it in the long term.
I detest this method but reading romance books can sort of help you in how to act towards girls if these girls are your stereotypical types that like eating lovey-dovey novels. Although they may seem crap, it gives you an indication on what how girls react and in some cases (for me anyway) I got caught reading a romance novel and my female friend and I had a debate whether romance in novels can translate to real life. Needless to say, we discussed more about books itself and that helped bridge the gap between me and her (No romantic feelings for her what so ever)
And with being the nice you.. I guess that's a start since being friendzoned isn't really that bad (To me anyway) it helps you to talk to the opposite gender which you can translate those skills to other females who have the potential to be with you. Plus you can always ask them for help too since they're roughly the same age because finding the 'one' isn't solely restricted to high school. University* is also a good place to look at since it's a fresh start and there's more
variety *With the uni one, I was at the bus stop at Monash Uni Clayton and while waiting, there was a Caucasian dude hitting on a International Chinese lady in Chinese. They eventually got in the same bus as me and went back to his place.
But don't worry about it now though.. focus on VCE because that's your future.