I was re-iterating what others were saying, guess I need to read more comments.
However, I would additionally recommend some books too. They helped me out a great deal. Some of the stuff it talks about seems so simple once articulated, but It blew my mind.
this book in particular Even if you never want to draw like he does, his observations and insight into what drawing really is about, not only made me a better illustrator, but opened my mind to a different perspective all together.
Now I understand. That's not bad for 15 year old work.
Read this book: Ryder's Complete guide to figure drawing
Practice your Agrippa until it's perfect.
Figure out shadow cores.
And stop drawing skulls, They're unique now but you're gonna get to college with a bunch of other kids just like you and it's going to turn into a skull drawing contest. It's actually half the reason I switched to sculpture.