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 ;)