My DS has a pine nut allergy (thankfully not anaphylactic but still pretty severe) and like a lot of others I used to think so long as he didn't EAT pine nuts he would be fine.
DP and I sat down with a bowl of pasta with some pesto and kept it well out of reach of DS. Low and behold, 10 minutes later his lips were swollen and he was covered in hives. I think he must've just touched us... I honestly have no idea how it happened but I'd imagine with kids, their bad hygiene and their propensity to touch everything it would be a nightmare for parents, children and teachers to have a 'nuts OK but no sharing' policy at school.