G.H.
Start off with asking her what she thinks 'sex' is. When I was about 13 the boy I liked asked me to 'make love'. I said yes thinking it was just kissing & maybe some touching. Boy was I wrong and way to scared so nothing happened.
Also ask her about this boy & what she likes about him & why she wants to have 'sex' with him. If need be seek the advice of the school counselor for advice.