• 30 Days of Mindfulness in the Classroom Guidebook 
• Quick and Simple Mindfulness Activities for Kids in the Classroom
• The Mindful "Feeling the Music Game"
  

For elementary students: 

• Classdojo has a video series on mindfulness.
• This Edutopia article has 8 activities for students.

For secondary students: The following are links to several videos highlighting specific mindfulness practices that are 5 minutes or less and can be used as class warm-ups: 

• Box breathing
• Body scan
• Heart-centered breathing
• Five senses grounding exercise
• 1-minute mediation
• 5 minute-chair yoga
• Gratitude notes
• Brain breaks
• Cleansing breaths
• Finger tapping