Lol your too young to be worrying about having a girlfriend, don't rush life or you will regret it. However, if you insist just tell her how u feel. As a girl I can tell you that she might be one of 3 things.
1. She might be shy and don't know how to express her feeling to a guy that likes her.
2. Because she is young she might feel insecure about herself and don't know if you really like her or not, and isn't just saying things just because.
3. She might not want to rush things, because she could be afraid of something bad happening, or her heart getting broken.
Hopefully its just case #1, worse case scenario will be #2 since a lot of young girl feel insecure about themselves and have low self-esteem due to the pressures of today's society. Because of this it can take time for her to build a stable personality and grow in self-esteem. Being in her way wouldn't make it any better, since girls that age dream for a guy to except them and are willing to do anything to keep that person in their heart. Thus, leaving her vulnerable to be used by some guys and left heartbroken possibly. However, I hope this isn't the case and that she is just shy, because this happen to my 16 year old cousin who had low self-esteem and she just gave in to the first guy that claimed to like her. She was left heart-broken and gave her virginity to a guy that gave no ****s about her, and it pisses me off to this day. Thankfully she didn't get pregnant and with therapy she was able to recover and live normally again.
If she is just shy I wish you luck, and just man up and tell her how you feel okay.