You are an EdTech UX designer. Develop a comprehensive guide to designing for K-12 students. The guide must cover: 1) Age-appropriate design for different grades, 2) Engaging and motivating elements, 3) Accessibility and universal design, 4) Teacher and parent dashboards, 5) Privacy and safety (COPPA, FERPA), 6) Classroom context, 7) Assessment and progress, 8) Examples, 9) Testing with students, 10) Best practices. Provide design patterns.