Police had no immediate indication who had thrown the block or why.
The school principal, Noreen Campbell, said the block struck one 15-year-old boy but didn't seriously injure him. The other six students suffered minor cuts or shock, she said.
The students attend north Belfast's only religiously integrated high school, Hazelwood, whose students suffer harassment by hard-line children from both sides.
The attack on the Protestant Skegoneill Road followed a second night of rioting by Protestant militants in the nearby Crumlin Road district.
A Northern Ireland police chief says it's the worst violence he's seen in decades. The clashes on Wednesday and Thursday nights involving several hundred men and teen-agers hurling gasoline bombs, firecrackers and rocks have wounded 46 officers preventing the mob from attacking nearby Catholic homes. A Protestant woman was also shot in the leg Thursday night as unidentified gunmen fired sporadic bursts of live rounds at police lines, forcing officers to scramble for cover but hitting none of them.
"What we are seeing is utterly reckless," said Chief Constable Ronnie Flanagan, commander of the Royal Ulster Constabulary, the predominantly Protestant police force.
Flanagan blamed much of the violence on members of the Ulster Defense Association, an outlawed Protestant group that is supposed to be observing a cease-fire in support of Northern Ireland's 1998 peace accord.
He said Thursday night's violence must have been organized. "You do not have spontaneous access to 100 petrol bombs," he said.
During more than three hours of mayhem on the Crumlin Road, militants hijacked and burned four vehicles. Police said they responded with eight plastic bullets, snub-nosed cylinders that are designed to knock down rioters.
Since the rioting began Wednesday night, police have said they were preventing Protestants from marching toward Catholic homes in the nearby Ardoyne district, the focal point for regular confrontations since June.
The Protestants have denied they wanted to attack Catholic homes and blamed heavy-handed police tactics for provoking them. Catholics have complained that the police weren't being hard enough on the Protestants.
Before Thursday's violence, police searched scores of Protestant homes and pubs for weapons. They found a rifle and two handguns at one address, but arrested nobody.
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