Keep it concrete and relevant: Use story and anecdote rooted in your own and their own experiences and routines that young people can relate to. I learned with practice to avoid the tendency for mindfulness teaching to become too abstract, too wordy and too didactic. Use everyday incidents from your own life and the students lives to illustrate mindfulness concepts. Relate it to sports, parents, grades, friends, and other issues they deal with day to day.
Build trust first: Most of the early sessions I do with a class are about establishing trust with the students. They have to feel safe and that they can trust you before they will engage in a meaningful way with you. That's because anything that requires them to share their own experience, no matter how simple or superficial can seem risky to them. If you do not have the trust of the students then they will not really engage with the exercises. This may take a few classes or even a few weeks.
Create variety and movement: Students are being lectured at all day. So find opportunities to keep the exercises interesting by allowing them to stand, move around or "shake out" some of their restlessness for 15 - 20 secs between exercises. Sometimes the time of day will influence energy and engagement levels of the class. Be aware of this and work with it creatively. I think best time to teach is mid-morning and with engagement lowest just after lunch or mid-afternoon. Use a variety of exercises that explore the full range of mindfulness activities (seated meditations, mindful eating, listening or gratitude practices).
Give special help to students with learning disabilities or anxieties: My experience here is that these students might have shorter attention spans or some cognitive challenges. Some may struggle with english as their first language. I focus on talking clearly, talking a bit slower and using shorter exercises at the start. Often I will check in more frequently with these students that they are following me OK. You may need to work through your mindfulness curriculum at slightly slower pace and even double back on any exercises that were either a success or more challenging.
We are planting seeds: Not all the kids are going to be fully engaged all the time or even most of the time. Expect motivation and engagement and interest to vary week to week. Class engagement may also be influenced by other school or class related issues you may not be aware of. So if some sessions are harder than others to engage students, it may be due to factors outside your control. So always be aware we are not trying to make the students "do" something. There is no pass or fail. There is no ideal end result. Rather each session is merely an invitation to explore a new way of being with themselves (and each other). And we are planting seeds that may one day bear some fruit for students when they most need it.