How could i make this girl get serious?

well theres this girl at my school and me and her have ben hanging out and talking and kissing alot but when i asked her out she said we should stay how we are, and i really like her and she says she likes me the same way but she wont kiss me in front of too many other people except the people in our homeroom and its really annoying and she gets mad when i do anything with other girls so i cant seem to figure out where i am with her, what should i do