Is it permissible for a teacher to go on strike to improve his financial situation?

If the employment contract is valid, it is not permissible for the teacher to cease working to demand a salary increase, due to the obligation to fulfill the contract, as Allah Almighty says: "O you who have believed, fulfill [your] contracts," and as the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions." However, if the contract has ended, he may demand a salary increase before renewal, and he then has the choice.