Years ago this happened to us on a school trip, but it was a pupil. She had been born overseas somewhere and didn't have the correct visa for France. They kept us for ages in the coach and kept rechecking everyone's passport and being really suspicious. The poor girl's dad hadn't realised that we were travelling overland and had only got her a visa for our destination country.
It was really frightening and in the end they had to refuse her entry so she and our terrifying German teacher had to get off a coach at two in the morning and just... Find somewhere to stay in Folkestone.
I can't imagine in this climate how scared those kids and staff must have been for their teacher and embarrassing for the poor man who probably checked and rechecked and did everything right.