Is the salary received unlawful, given that the number of classes taught is less than stipulated due to student absenteeism? And is it permissible to give a portion of it as expiation or to dispose of the unlawful gain, and what is the required amount to be given?

An employee or a hired worker is entitled to their wages by presenting themselves and being ready for work, whether they actually perform the work or not. If a female teacher comes to school but the students do not show up, or only one student attends and she refrains from teaching him to avoid being in seclusion (khalwa), she has fulfilled her obligation and her salary becomes permissible for her. If it is possible to teach the student in an open room or in the administration office, that is better than canceling the lesson. It is preferable for a woman to limit her teaching to girls only, or to abandon teaching altogether if that is not possible.