My general philosophy is that if I want 2 students to work together, they get on with it regardless of whether they are friends/like each other/whatever. With most groups/students this is fine and even if they don't like it, they get on with it. Especially younger years. But year 10/11 are so entrenched in their friendship groups that if I ever ask them to work with a different student, they can sometimes kick off and refuse. I have thus far taken a zero tolerance approach, suggesting that everybody has to work with people not of their choosing, and it's a fact of life so deal with it. Am I being unreasonable? Are they at an age where making them work in this sort of situation can cause real stress?