Depends. Plenty do for such reasons as
A. There is cctv and patrolling staff in the playground, but not in the building.
B. They have a fundamental moral objection to being told what to do.
C. It's fun getting teachers and other staff into trouble if you know they get it in the neck from SLT.
D. It's fun showing a member of staff that they have no choice over whether s/he gives you attention or not.
E. It's a way of showing others that you're the teacher's favourite because she lets you come in and spend half of her lunchtimes with her. The teacher breaks the rules for you because you're special.
F. You've got a stonking crush/fancy her and think that if only you could spend some time alone without other kids around, something would develop.
After all, if this were a girl and a male teacher potentially alone together with no normal means of anybody seeing what could be happening, spotting her in the corridor every lunchtime and she was saying 'I just don't want to be late', would everybody still be comfortable with it? I'd certainly hope that SLT would make damn sure no teacher was put in that position/able to do such things unchecked and no child was able to be at risk.