How borders are drawn and enforced has far-reaching consequences, whether we live on either side of them or halfway across the world.

A middle school physics exercise caused a storm in Poland, after reports surfaced that a teacher asked his students to calculate how many Syrian refugees they would have to push off a raft, in order to reach Greece.

The teacher, Grzegorz Nowik, dictated this problem to private school students in the town of Białystok, back in September:

Four Syrian refugees are trying to get to Greece on a tiny raft with the dimensions: 1m x 2m x 20cm and 800kg/m (39″x 79″x 8″ and 547 lb/ft). Calculate how many refugees you’d have to push off the raft for them to be able to get to their destination if each weighs 60 kg (130 pounds).

A concerned parent of one of the 14-year-olds immediately took the exercise to the school’s principal, who reportedly warned that if it happened again, his contract with the school would be terminated. According to the deputy principal, Nowik was remorseful and apologized. She told Polish TV station TVN24 that the school emphasizes teaching openness tolerance: “We have so many various projects about discrimination and equality that our children are open-minded.”

But some dissatisfied parents expressed their outrage about the incident on social media. The story was picked up by mainstream Polish outlets this week with headlines of a “shocking exercise” and “scandal.”

“This is no explanation, but sweeping the problem under the carpet….the school should have organized a discussion about the situation of refugees, perhaps invite some of them as well,” one parent told Polish daily Gazeta Wyborcza.

Nowik has tried to explain that the problem was a joke, meant to entice the students. “If I spoke about the lift of a block submerged in water, how many people would I engage? Maybe two,” he told the newspaper.

“After the lesson I realized this was not the best idea. I told the students it was only a joke, that in no way does it portray my attitude toward refugees. I don’t have anything against them—if it were up to me even five million could come here if they wanted.”