Is the salary earned from working as a music teacher in a public school permissible (halal) or forbidden (haram), especially since the music subject is not taught, based on a fatwa permitting the taking of wages for holding time?

Teaching music is wages for an unlawful benefit. If the contract with the government school stipulates teaching it, then it is obligatory to abandon it. However, if it is not a condition, then there is no obligation to abandon it. As for the wages taken for teaching it, they are wages for an unlawful benefit, not deserved by the contract, and the way to dispose of them is by donating them as charity or spending them on general Muslim welfare. If the poor person is in need, it is permissible for him to eat from it according to his need.

As for the teacher remaining in his position to reduce evil, this is acceptable if he is diligent in repelling corruption and his remaining in it is better for Muslims than someone else. There is no sin upon him for remaining and taking, and he will be rewarded for his effort in reducing the evil.