Hio! Thanks for bringing up the topic!
So, first, as I stated in my "popularity" post, you being "not so popular", is not true. True popularity, is if you have a kind heart, willing to help others, generous, etc. Sure, you may not have the "big name brands" of clothing they wear, or you might not know about all the "hottest celebrities", but really, that's what Satan wants for us, to have a "big ego". Caring about others is important, slowly, people will start to like you for who you are, and being you is very important, if you follow all the things that the popular kids do, you're really just turning into someone that you won't be able to recognize soon. At my school, popular kids gossip a lot, they don't do the things that would apply to common sense, and so on. Being popular isn't a great thing in life, necessarily.
Second, you stated that you have a friend that only pays attention to you after school, there are certain situations surrounding that topic.
1) Does your friend only want to be friends with you for attention, and drama? A situation like this would be if she rants, and rants about how terrible her day was, and doesn't listen to what you have to say about your day, she fights for no reason-- and not just normal fighting, I'm saying fighting daily, and so on.
2) Perhaps you just need to see each other more often! Maybe during lunch break if she attends the same school as you, or perhaps if she lives in your neighbourhood, stop by her house at a reasonable time, and just visit with each other. A true friend is one who can help you when you're feeling down, stand up for you when you're bullied, and so on.
Third, you stated that you were really shy, and wouldn't talk to people unless they were your friends, I have a few tips.
1) If you have a few good friends, try making a few new friendships! Get to know the people in your class.
2) Become a confident person, stand up tall, with pride, if you walk around the place with a negative attitude, you can only expect to be treated that way, if you walk around with a smile, positive outlook, you'll be spreading happiness in no time! :)
3) Don't bring yourself down, if you didn't do so well on a test, or had a bad day, making yourself feel bad is also going to damage your self-esteem, for example, "Oh, I'm terrible at math.", will only make you feel worse, try this, "I'll do better next time!", will help a lot.
4) Smile! Even if you're having a rough time, fake it 'till you make it! Smiling will make you feel confident in yourself.
Hope this helps, God bless!