An online petition has been set up and a pan-African women's organisation is mobilising to get the president to apologise and reverse his comments.
Mr Magufuli warned schoolgirls at a rally on Monday that: "After getting pregnant, you are done."
A law passed in 2002 allows for the expulsion of pregnant schoolgirls.
The law says the girls can be expelled and excluded from school for "offences against morality" and "wedlock".
Women's rights groups have recently been urging the government to change the law.
