Annie, what cruise line? My responses are based on my experiences on Carnival, where most teen boys will wear dress slacks, dress shirt and tie and dress shoes on formal nights. Some, but not all, will have jackets. If your kids don't have one, I wouldn't go out and buy one. Don't panic!!! Have a great cruise.
Momx4---sounds like a good compromise to me! During the day, and after dinner, you'll see teens in all different kinds of clothes. In fact, during the day, they'll probably be in their swim trunks most of the time. It's good to suggest some new things, though. I did, and my boys ended up liking them; and......they look so good in their pictures! Don't stress; have a great cruise!