I like a this girl and i think she likes me,but?

I'm 15 and she is 14,I really like her and i think she likes me,for example,when me,her,her sister,and my sister, were at our horse lessen,our teacher said speaking to all of us"Ok you three girls come and help feed the horses,and nathan you and bill(the teachers husband)go and clean out the storage room"and the girl i like said"Oh can i help nathan instead?"and the teacher said yes.There are other examples but that's the best one for now.Now for the problem,she seems to subtly flirt with this other guy whose 21! PLEASE HELP!!!!