A Polish teacher has apologized after giving his students this physics assignment: calculate how many Syrian refuges would have to be pushed off a raft for it to remain afloat and reach Greece.

The teacher in Bialystok, Grzegorz Nowik, has faced criticism for the assignment he gave his 14-year-old students in late September. The incident only made headlines in Poland this week.

Elzbieta Stasiewicz, the school's vice principal, said he will not lose his job because he has expressed remorse and it was his first incident. Nowik told TVN24 he was just trying to make his lesson less boring.

Mostly Catholic Poles are largely fearful of Muslim refugees and oppose taking them in.

The incident comes as a summit of European and African leaders wrapped up in Malta, with world leaders seeking new ways to send migrants who don't qualify for asylum back to their homelands.

But the gathering quickly became overshadowed by fears that one of Europe's prized benefits -- the ease of travel through its Schengen passport-free area -- was unraveling.

"Saving Schengen is a race against time," EU Council President Donald Tusk warned.

He cited individual moves by Germany, Sweden, Slovenia and other EU nations in response to what they see as threats to their border security from the tens of thousands of asylum-seekers who have been streaming in from Africa, the Middle East and Asia.

"Without effective control of our external borders, Schengen will not survive," Tusk said. "We must hurry, but without panic."

The Schengen travel zone involves 30 nations, including some not in the European Union.

The Associated Press contributed to this report.