Very nice drawing, particularly for one drawn in class at school! You've got your proportions beautifully, so the only thing I will suggest is that you need to add more 'depth' in the shading. So the area's that have the darkest shadows, make them almost black, you need to spread the tones throughout your work so that it does not look weighted in one area. I tend to put the very dark points through sections of the ear, the eye, the nostril, mouth and often in small areas under the mane and jaw.
Just a little more fine details around the nose and the eye and you will have an EXCELLENT drawing on your hands.
No for the art class subject. You say not having lessons is holding you back. Well I can promise you that once you find your own style of work, having art classes will probably hold you back, not vice versa.
I tried to have art classes when I was about 12, and hated it. Then I did 2D art all through high school, and although I enjoyed trying out different mediums and techniques, I felt as though they were trying to shape my work into something it's not.
I pride myself on being a self taught artist, no body showed me how to draw, they simply showed me other mediums to try. But in the end, no body can really teach you art, they can simply guide you. Art is a very personal and emotional thing that cannot be forced. Once you find your own way, you will see what I mean ;)
I am proud to be self taught, but I would like to have a semester of art class to learn new techniques, even if I choose to ignore them. Even just to have the art teacher to look over my pieces wouuld be nice...and having the art supplies at my disposal would be nice! Plus I know the Drawing&Painting class have a unit where they draw skeletons and the little mannequins to get proportions down...I'd like to do that, since eventually I'd like to draw people. I've drawn four people, and they weren't very good. I totally understand what you mean about being held back...I think that I will get frustrated in class if I am told how and what to draw, but if it makes me a better overall artist, then I'm willing to do it. I'm 15 and still learning...I feel like I just need some instruction, ya know?
I don't like critiqueing art as such, but you ould try being a bit looser, maybe getting a 5B or 6B pencil, and just relax a bit more.
Otherwise it's very good, you have all the shapes right, which is the hard bit!
It took me a lot of practice and experimentation to improve.