Teachers are protesting the suspension of the union leader of local UN employees who appeared alongside the Hamas prime minister of Gaza at a public event, said Yousef Hamdona, another leading union member.
The UN Relief and Works Agency in September suspended Suhail Al-Hindi for three months because it bars local employees from engaging in political activity.
The agency says its 243 schools serve 219,000 students.
The strike is to last through Thursday, the union said. Teachers have staged two other related walkouts over the past month.
Hamas, which violently took control of Gaza in 2007, has strained relations with UNRWA, which provides services, cash and food to hundreds of thousands of Palestinians in the impoverished territory.
But some in the Islamist movement accuse the agency of promoting a Western way of thinking in its schools and summer camps.
