Help: A high school student with B2 visa and how to extend the s

My niece came US in August and enrolled in Public high school since she has valid visa and can stay until next Februay. Now she wants to stay here to finish high school. I think I need to transfer her to a private high school for I20, but her stay will not be legal until next February. The private high school doesn't accept application (issue I-20) until next May. She has 3 months gap. What is best way for her to continue next semester study in US? One option is get a I20 from a Language school and transfer her to a F1 and the other one is to go back China during Christmas and come back for six more months.

Many thanks for your advice!