They teach antisemitism and terrorism.

I've found this article that talks about it (among some other reasons), though I've also seen it on other sites.

A recent report by the watchdog group IMPACT-se highlighted that UNRWA's educational materials, based on the Palestinian Authority (PA) curriculum, contain antisemitic content and celebrate violence and martyrdom-jihadist culture.

UNRWA has a significant position in educating almost 545,000 Palestinian children across the Middle East.

The IMPACT-se report examined educational materials used in UNRWA schools, particularly in the Gaza Strip, the West Bank, and East Jerusalem, where the PA curriculum is adopted.

The PA curriculum, which has been under scrutiny since its 2016 revision, is found to contain – like it did before its revision – antisemitic content and the promotion of violence, jihad, and ‘martyrdom’ culture, while omitting teachings of peace and coexistence. UNRWA, while not producing its own curriculum, supplements the host country’s curriculum with additional materials. Despite pledges to counter-act calls for violence by UNRWA, the report found a disturbing failure to do so.

More than 80 UNRWA teachers and staff across more than 30 schools have been caught distributing hateful content in their textbooks.

According to UN Watch: “Teachers and schools at the United Nations Relief and Works Agency, which runs education and social services for Palestinians, regularly call to murder Jews, and create teaching materials that glorify terrorism, encourage martyrdom, demonize Israelis and incite antisemitism.”