Centre principal refusing to give a normal chair to sit.

I am a pre school teacher and 8 weeks pregnant. My centre principal is refusing to give me a normal height chair to sit on despite memo from gyne to refrain from sitting on a low chair meant for PG and Nursery 1 children. She is insisting that I have to use the low chair or if not I can take hospitalisation leave or no pay leave till I am fine to sit on the low chair, when she can simply put in the older children class where they use the normal height chair. She claims what if the children comes and hugs me and something happens to me which I feel is not reasonable at all. Because of this I am on my feet whole 9.5hours and cause my calves and feet to swell and painful. Sometimes I causes cramps in my lower abdomen too. What can I do?