I lied to one of my best friends?!?!?!?

okay well i have this friend tht goes to a diff highschool then me, and well she was kind of shy wen i met her 2 yrs ago, and so she is a year older then me but wen i met her i told her tht i was her age, and now shes really popular and everyone loves, her and my cousin is like bffs with her, and im like afraid she'll figure out i was lying to her for so long, by talking to my cousin and im shure wen they get on the topic theyll figure out ive been lying. and well my friend has been telling everyone im a senor even tho im only a junior. even tho in reality im really supposed to be a senor bc i missed a year and went to kindergarten a year late. but thts not the point. how do i tell her i was lying to her w/out seeming for it to be so bad, even tho ive kept this against her for 2 years!!