It is singular, (leg) pulling their leg, pulling your leg, pulling my leg, pulling his/her leg.
You are correct, except that you should have said in your example above pulling their legs, as the use of 'their' refers to more than one person, as Pawian was when he was talking about his students, not one student. Or do you think students have only only one leg between them? Aside from this, the whole thread is about Polish idioms translated into English. Pulling someone's leg was not an idiom for people to guess the meaning of, but you knew that anyway. Another attempt to trash a great thread.
@ Pawian, I answered your last riddle in post #440
ask her to make it at an age when we are fully capable of making them ourselves.
Yeah, I guess we all were guilty of trying to wriggle out of doing the boring stuff as kids!