Is it true that one who falls short in the teacher's secondary duties does not sin with regard to his salary, because these duties do not have a specific financial value within the salary, and that the entire salary is in exchange for teaching students?

The matter refers back to the nature of the contract concluded between the teacher and the hiring entity. The teacher is bound by what the contract stipulates or what custom dictates. Anything beyond that is not binding upon him, and he bears no responsibility if he fails to perform it. However, if he fails to perform what is binding upon him, he does not deserve remuneration for the work he did not carry out. The ruling on this depends on knowing the terms of the contract and the custom in the country.