Pay of HMs in bilingual / H.S. schools

There are few aided schools where two separate units based on medium of instruction are running simultaneously – Bengali & Hindi. Teachers are also appointed separately through SSC. But, HM & AHM are the same for both the units. Here HMs & AHMs are entrusted with additional duties and responsibilities because of difference in medium of instruction, compared to their counterparts in monolingual schools. Why don’t HMs & AHMs of bilingual schools get additional financial benefits? Similarly, why don’t HMs & AHMs of H.S. schools get additional benefit compared to their counterparts of secondary schools?