Slurs are usually not popular with students, but indispensable to build embouchure strength, range, and breath control. Start with easier downward slurs, then introduce upward slurs, assuring the student that slurs take time to master. While lips are resting, challenge students to name the notes that can be played in a certain position. Don't demand legato playing too early. This requires near perfect synchronization of tongue and slide.
Methods ask for slurs early on which is fine for trumpet players, but too early for trombones.
When doing this drill, make sure students are using tongue, not just breath attacks.