Working as a dance teacher for women
Question

                What is your opinion of a place in which only women work, teaching dance to women, e.g., ballet and other types of dance, as a hobby and not as a profession? What is the ruling on money earned from this profession (dance teacher)?  Please note that the training will be done to music. If this income is haraam, what should one do?
Praise be to Allah.It is not permissible for a Muslim who lives in a Muslim land to doubt the prohibition of working in places where women dance, ‘awrahs are uncovered and evil acts are committed. The prohibition on such things is well known in Islam. If such actions are not haraam, then what is haraam?? 
Undoubtedly the one who exposes himself to such actions is only exposing himself to the anger, wrath and punishment of Allah. How can he dare to commit sins such as these openly in front of people, and even help to propagate them, teach them and train others in them, when he knows deep down that this deed is inspired by the Shaytaan and he is aware of the abhorrent nature of some Western practices which involve nakedness and promiscuity. Allah, may He be glorified and exalted, may give respite to the evildoers but never lets them off, and He has prepared a severe punishment for those who propagate immorality and corruption in the land. 
Allah, may He be glorified and exalted, says (interpretation of the meaning):
“Did you (O Muhammad (Peace be upon him)) not see (thought) how your Lord dealt with Ad (people)?
Who were very tall like lofty pillars,
The like of which were not created in the land?
And (with) Thamood (people), who cut (hewed) out rocks in the valley (to make dwellings)?
And (with) Firaun (Pharaoh), who had pegs (who used to torture men by binding them to pegs)?
Who did transgress beyond bounds in the lands (in the disobedience of Allah).
And made therein much mischief.
So your Lord poured on them different kinds of severe torment.
Verily, your Lord is Ever Watchful (over them).”
[al-Fajr 89:6-14]. 
These matters are very well known in Islam and there is no need to establish proof concerning them. The one who approves of them or regards them as permissible is in grave danger; we ask Allah to keep us and you safe and sound. 
As that is the case, the income earned from teaching women how to dance half-naked and from participating in the promotion of corruption and sin in the land is haraam earnings. It is not permissible for him to consume it or take it or use it; rather he must hasten to repent from that. 
It says in al-Mawsoo‘ah al-Fiqhiyyah (23/10-11): 
If the dancing is accompanied by something haraam, such as drinking alcohol, uncovering the ‘awrah, and so on, then it is haraam according to scholarly consensus. There is no difference of opinion among the fuqaha’ that it is not permissible to rent out property for haraam purposes. As dancing is haraam, it is not permissible to rent out property for that purposes.
End quote. 
Repenting from acquiring wealth that is haraam because of the manner in which it was acquired is done by giving it up and donating it to charitable causes. So he should give in charity all of his wealth that he acquired by haraam means. But there is nothing wrong with him keeping what was produced from that haraam wealth by means of permissible trade, or keeping enough to meet his needs until he finds permissible work from which he can earn a halaal income. 
We have previously discussed how to repent from haraam wealth in the answers to the following questions: 78210, 78289, 81915.
And Allah knows best.