In numerous girls' schools in the Palestinian territory, which reopened their doors on Sunday following the summer recess, students were told they would be sent home if they do not conform with clothing regulations.
"We invite all schoolgirls to wear the jilbab," a senior official in the Hamas education ministry, Mahmud Abu Hassira, told AFP. The jilbab is a traditional, long, loose-fitting robe worn by women in some Muslim countries.
Yet, the Islamist movement insists it did not order girls to wear the jilbab but let school principals know they should no longer ask students to wear the old uniform that consisted of a long blue skirt and matching blouse.
Hamas government spokesman Taher al-Nunu stressed that the decision to do away with the old uniform was taken "in order to alleviate parents' financial burden caused by the blockade and the asphyxiating economic situation."
Israel and Egypt have sealed off the Gaza Strip from all but vital humanitarian aid since Hamas seized power there in June 2007.
Many of the girls in public schools could be seen on Monday wearing the jilbab and a headscarf, which are considered symbols of modesty in much of the Muslim world.
Hamas has also decided that only women can teach in public schools for girls.
"Our society is Muslim and Islam imposes the separation of sexes from the age of seven. We have acted on this basis to replace male teachers with female ones," said Mahmud Abu Hasira, a senior official in the education ministry.
The moves were widely seen as the latest attempt by Hamas to impose conservative Islamic practices on the territory's 1.5 million residents.
Hamas had earlier ordered women lawyers to wear headscarves in court. Residents say the morality regulations extend to beaches where women are expected to be fully covered, and men must wear knee-length shorts and shirts.