The only way to really learn improv is to do it. If you do too much verbal coaching, your students spend their time in their heads, not on their feet. If you're trying to make a point, have them get on their feet and do it/show it. Even better...
You Need To Improvise Too
This comes in two forms. The first is not being afraid to get on your own feet and perform with your students. Don't be the omniscient voice in the back of the room or the side of the stage. Let your students see you play with them. You can better guide a scene or illustrate a point that you're trying to make.
The second is in terms of the actual flow of the workshop or class or rehearsal, don't have a set agenda. Chances are if you've ever started a session with a pre-set plan, somewhere along the way you went off the plan, maybe by a little, maybe by a lot. Instead, respond to the needs of the group. Have an overall theme for the session, and a big set of games and exercises to draw from, but let the group help determine the course of the session.
You're An Expert, Not a Know-It-All
You have the knowledge to guide your class, but you probably don't know everything. When a question arises, don't be the first to answer - ask the class. It often generates much better discussion than listening to you yak on and on. Occasionally you'll get a question to which the answer is clear - but most of the time you won't. Those are the best questions.
Set The Ground Rules
What are your expectations when it comes to folks behaving in class? No cell phones? Respectful attention during scenes? No profanity? Miss no more than one rehearsal? Everyone's opinion deserves to be heard?
Make sure you set those very clearly from Day 1, and expect to have to continue to reinforce them. Make sure that students know what's appropriate and what's not. It's particularly difficult when it comes to high school troupes - the line that they're not allowed to cross tends to me more restrictive that with most other troupes - but training them develops their improvisational muscle memory.
Encourage Questions and Comments
Provide an encouraging learning environment for your students, one in which they feel comfortable asking questions (not just to you but to the whole group) and sharing comments. Let students learn from each other - if they're not sure of something, let the group provide clarification. This is especially important if you're working with just a performance troupe - if they don't trust each other to ask questions and share their feelings, then there's something crucial missing from their chemistry.
Go See Other Improvisers
Improvisers, particular high school students, can get stuck in a rut if they don't get a chance to expand their view of improv. Encourage your students to see other shows or other formats. Take them to a neighboring school's improv troupe's show. Get your short-form-focused students to a professional long-form show. After the show (immediately, if possible), discuss with them what they saw. What worked, what didn't, what looked like us, what didn't. Get them to think outside their box.