I did my work experience in my L6th year back in 1998 at Heathrow T4 for BA. It was great fun, and we even got a temporary airside pass too, we had to be escorted at all times though. There were a couple of other people from my school that got to go too. By the end of the week they had us checking in the economy pax! They let us watch at the FIRST/Concorde check-in and we even got to use the Concorde room! The security people even had us checking people's passports on some of the flights. Back at school the next week everyone else was moaning about how they were so bored and had to make the tea for everyone, that week was certainly far from boring. It certainly made me appreciate how hard all the staff work there.
I'm not sure how the school arranged it, but it was during the time that Bob Ayling was in charge, and he was an old boy, so I suspect that played a signficant part.