You don't have a boyfriend probably because you haven't met someone who you will like as much as they will like you. You are still young and therefore you shouldn't be worried about getting a boyfriend just concentrate on your studies. You will get a boyfriend when the time is right.
Many things could contribute to this.... and its not being "ugly" at all so get that thought out of your mind. One reason is it could be that you aren't very open, personality wise i mean. Start doing things that he likes, ik its what everyone says but really it works. Be yourself and have fun. That is what really attracts guys. Them seeing you being able to be comfortable and having fun anywhere will want them to be close to you. I only speak from experience.