Forthan, a McDonalds' All-American post player who has signed with LSU, scored Georgetown's final 10 points, including two free throws with 21 seconds left.
Forthan spent last season playing junior varsity after the University Interscholastic League ruled she had moved to Georgetown from Portland, Ore. for athletic reasons and declared her ineligible for varsity play.
Forthan was 11 of 24 shooting, including 3 of 4 on 3-pointers for Georgetown (33-3).
Elsik (31-6) had a chance to tie with 27 seconds left but Jazmine Ocanas' 3-pointer rattled out. Crystal porter scored 16 to lead Elsik.