Middle East observers felt a sense of deja vu Tuesday as Israeli tank shells slammed into a U.N. school near Gaza City, killing at least 30 Palestinians who had taken refuge there from the war raging around their homes. In 1982, a massacre carried out by a Christian militia group at the Sabra and Chatilla refugee camps killed hundreds of Palestinians; the camps had been under Israeli control. In 1996, there were 118 people killed at a U.N. compound by Israeli artillery in the town of Qana, and in 2006, an Israeli air strike killed 56 in an apartment complex in the same town. All three events spelled the beginning of the end of Israeli campaigns into Lebanon. Until now, the international community - including Egypt - has given Israel a long leash to strike a heavy blow against Hamas. But with the shelling of the U.N. school in the northern Gaza town of Jebaliya on Tuesday, the clock might start ticking for Israel to withdraw its troops. Dr. Bassam Abu Warda, director of Kamal Adwan Hospital, told reporters that 36 people were killed in the strike on the school. The United Nations confirmed that 30 were killed and 55 injured by tank shells. Gerald Steinberg, professor of political science at Bar-Ilan University in Ramat Gan, said the strike against the U.N. school and ensuing diplomatic pressure it could create were not only foreseen but discussed in advance by Israeli policymakers. There were many meetings on how to deal with this before the fighting started. It happens every time," said Steinberg.