How do I ask a girl to give me chance to get her on a date?

[Vex]

Good day,
I am 15-16 years old and I need an advice.There is a girl in my class who I really like.We have been friends since 1st grade but not really close ones,we were hanging out sometimes with other guys and girls from class.Now,whenever we go out with our friends she would be hitting on me.Not much but she would laugh at any of my stupid jokes,hitting me with her leg when sitting etc.The point is that,were were chatting on Facebook recently and I asked her out.And she said yes.But a few days later she said like "Well we can go out sometimes,but,I don't know,like a friends maybe." and after that we met at the party in my friends place and she was again,flirting with me.Now,I am really confused,what is going one here xD So I thought to send here a letter or text saying something like to get her on a date again or maybe something else to get her back.I really need help I don't want to sound like a "Looser" to beg her to go on a date with me,or I don't wanna be like "I won't stop till I win you",don't wanna sound like a "Pervert,or Reaper"....Can someone please help me with this I would appreciate it.Thank you!