If one of the signs of puberty appears in a Muslim girl, she is obliged to wear hijab, do all the obligatory duties and to avoid all haram things.

Women in ihram and otherwise are obliged to cover their faces in front of non-mahram men, because the face is the center of beauty and it is the place that men look at… and Allah knows best.” (Fatawa al-Marah al-Muslimah, 1/396, 397)