I'd like them to hear that their unique perspectives are valuable and worthwhile. And that the only way that other people are going to benefit from their unique perspectives is if they share them, either through writing or speaking or even asking questions. When they sit there without saying anything day after day, they learn little and they share less.
I'd like them to hear that their learning will be valuable to them, if they learn well. If they do only the bare minimum of studying and writing and doing homework, then they'll learn the bare minimum and they'll have barely minimal value to offer to others in terms of skill or knowledge.
I'd like them to hear that the habits they build now are going to stick with them for the rest of their lives. Why not build positive, fulfilling habits?
I'd like them to hear that they're special, and take that specialness to heart. If they treated themselves as something special--as someone deserving of respect--then they wouldn't be able to help treating others that way, too.
There are other things that I'd like them to hear, but this is a good start. After all, we wouldn't want to overwhelm them with too much stuff, would we? If I could just get my students to hear some things like these, then we'd be well on our way to helping the world to be a better place.