All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
The intention of your brother to give the extra income of the school to charity and to the general welfare of the Muslims is a good intention, but Allaah is Tayyib (which implies pure), and He does not accept except that which is pure. Good intentions are not achieved by means that are not pure (or good), and what you have mentioned about the nature of the school activity involves violations of sharee'ah (Islamic Law). For example, the owner of the school is not permitted to allow women who do not wear the hijab (Islamic dress) and enable them to teach male students who have reached or nearly reached puberty or to mix with male teachers and so forth. Also, the person in charge in this school cannot allow dancing activities if it is accompanied by some prohibitions such as musical instruments, mixing between men and women, nudity (being scantily dressed), or men behaving like women (or displaying effeminate manners) or imitating them. What you have mentioned with regard to the national anthem being accompanied by statements contradicting Tawheed is a great evil, and it is not permissible to allow it. For more benefit, please 81239, 85728and 83792.
You described yourself as one of the directors of the school, so you bear part of the sin of those evils according to the extent of your responsibility in the school; Allaah says (what means): {And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allaah; indeed, Allaah is severe in penalty.} [Quran 5:2]
As regards the money earned from this school, what was earned in return for teaching permissible subjects is permissible, and the fact that the school includes prohibited activities does not make the earnings unlawful if such prohibited activities are not meant in the fees of the school.
Allaah knows best.