WOW! First off: amazing work for your age! I wish I were as good as you when I was 15...and I love your mentality "get as good as possible before college", I truly believe the sooner you get through the hard patches, the better!
Now unbiased critique: You seem to understand a lot of fundamentals already (light source interacting with planes, local color/value, texture...etc) but they don't seem fully developed yet (you are making some mistakes such as, in the last image: rendering too "shallow"/not enough contrast, some confusion about lighting (her left arm), value ranges too close together (horns/skin/hair/dress),etc...)
Now, I'm no pro, so dont take my advice TOO seriously.
I think doing some studies and applying them afterwards (! important !), like painting a portrait from a photo and then painting a head from imagination, would really push you forward (and will introduce you to a method of studying which can be applied to pretty much everything you would ever want to paint).
But more importantly, do these studies informed. Learn as much as you can about how light interacts with surfaces, how value models forms, colors interact, how images work... Don't wait for college to learn art theory (fundamentals, pratical stuff).
I'm sure you have been doing your own research, but here are some favourite books/ tutorials:
http://www.fusroda.com/resources/ (search for "Struggling with value - Marek Okoń") - about value ranges
http://imgur.com/eih4nLC - abou "shallow" values
http://www.alexhays.com/loomis/ (greatest books! A bit cryptic, but a goldmine of information!)
James Gurney "Color and Light" (MUST HAVE)
Michael Hampton's figure drawing book
And the list goes on, but I will stop now, it's too much hahaha
Good luck learning and hope to see you more often here!