Staff and pupils at more than 240 UN schools in Gaza began a five-day strike on Sunday in protest at UNRWA's suspension of a teacher for alleged political activities.
"Students and employees, including teachers, will strike from today until Thursday to protest over the unjust UNRWA decision to suspend Suheil al-Hindi," a union source told AFP.
It was the third such strike to hit schools run by the United Nations relief agency for Palestinian refugees in a month following the suspension of Hindi, a local teacher who heads the agency's Arab Workers' Union.
Hindi was suspended for three months in September after organising a union activity which was attended by a large number of officials of the Islamist movement Hamas, which controls the Palestinian territory, Gaza sources said.
The move was seen as a violation of UNRWA's internal regulations, which ban staff involvement in extra-curricular political activities without prior approval by the agency's management.
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There are 220,000 pupils studying at 243 schools in Gaza, run by the UN agency for Palestinian refugees and staffed by some 7,500 teachers and other school workers.
Teachers and other school staff first went on strike over the same issue at the end of September, and again in early October to demand that UNRWA reinstate Hindi.
But UNRWA urged the school staff not to drag "innocent children into their escalating campaign of industrial action."
"We appeal to the union to call off the strike which is damaging the education of our children in 243 UNRWA schools," said spokesman Chris Gunness, urging staff to "respect all UNRWA's policies, procedures and decisions."
"The union are playing dangerously into the hands of those who seek to defund and destroy UNRWA," he said.
"We urge the union to call off this action and put the refugees at the centre of UNRWA's agenda."
The UN agency works with around 4.8 million Palestinian refugees across the Middle East, a million of whom live in the Gaza Strip.