Al Jazeera's Ayman Mohyeldin, reporting from that school, said hundreds of students had gathered as Palestinians struggle to get their lives back to normal.
"A Muslim aid organisation has come and is performing songs and dance, trying to lift the spirits of the kids," he said.
Maher Wahba, a psychologist with Muslim Aid, said that one of the first tasks would be to address the psychological trauma being suffered by children who had lost family members and friends.
"We are here to let the children act out their stress and relive what has passed during the Israeli invasion," he told Al Jazeera.
"These children have suffered a lot, we have seen many cases, many psychological disorders ... aggressive behaviour, many nightmares, dreams."
Unrwa, which provides basic aid and services to most of the 1.5 million people living in Gaza, employs some 200 counsellors and is looking to recruit more in the wake of the war.
In Rafah, students from the Dar al-Fadila school attended lessons in tents set up near the rubble of their destroyed building.
"We will learn and grow, God willing, even inside the tents. This will not break our will," Fatmeh al-Bashir, one student, said.
The reopening of the schools means that the UN must now find somwhere else to house many of the thousands of Palestinians who took shelter in them during the Israeli bombardment.
"We will rent houses for them and we may put some in tents ... but at the moment we still do not have a clear plan or direction," Riad Radwan, manager of the local Unrwa shelter, said.
In Jabaliya alone, Unrwa says that 20,000 people had to take refuge in UN schools during the war.
Most have now gone back to their homes but others will be moved to another school building as their houses have been destroyed.
However, Hajj Abu Aysh, a 61-year-old father, told Al Jazeera's Zeina Awad that this was not an acceptable solution.
"They want to move us from school to school to school? Is this a solution? We are not animals," he said.
"Unrwa and the UN and the international community are obliged to solve our issue."