Is it permissible to receive the full salary when the student is absent, given the impossibility of replacing them with another during the period of absence?

If the agreement with the child's guardian is for a fixed monthly salary, and the child is able to attend but does not, then the full wage is due, whether the child attends or is absent, unless the absence is due to negligence on the part of the teacher. However, if the system of work and payment is per session, and the time or place was not reserved for the child, or the child was unable to attend due to the teacher, then the payment is only due for the actual sessions. The principle is that payment depends on work, and compensation is given for what is compensated, and "Muslims are bound by their conditions." Accordingly, the teacher is entitled to the full month's salary in the case mentioned in the question, even if the student misses some sessions.