Eat Out To Help Out.

It has a card with their photo, name and student number on it. It’s so they can’t refuse to give their name, or so you can sneaky see it without having to ask. They also scan it to pay for food (which is good as free school meal credit is just loaded on there so no one knows who is FSM and who isn’t, no cash is accepted, you load your money on the cards at machines in school or online), to borrow books out and to print. The lanyards are different colours for each year group so you can tell at a glance if someone is in the wrong place or not (it used to be useful to see if someone was skipping tutor and having two breaks but it’s especially helpful now with bubble areas).

Edit: all staff wear them too. Again, so everyone knows our name, to scan for food, books, printing and also to open doors. It’s also for, as you say, proof that it’s not a random person wandering in off the streets. The 6th formers can also open doors and they use it to scan in and out at a machine in reception as a register for when they have free periods.