Hi Colin,
Congrats on the potential student trip! I've been leading high schoolers on the Camino and other routes over the last ten+ years. It's a great joy and an awesome experience for the students. I hear where you're coming from on the evening concerns, and it's something I have in mind when leading other trips. It has never really been an issue on pilgrimage, though. Walk them hard enough during the day and they're tired at night!
I don't have any good suggestions for evening activities in Arzúa. It's a pleasant spot, but unless you're there during one of its festivals, I don't know that there will be anything notable to do. If you're concerned about this, though, one option you might consider is staying after Arzúa. When my group last walked the Camino Francés, we stayed in the Albergue Camiño das Ocas, located about an hour after Arzúa, in pretty much the middle of nowhere. We had a nice leisurely afternoon in town, eating lunch and hitting the supermarket, and then carried food to cook dinner together in the albergue that night. Worked out well!
Dave