It is likely that highly able students will lose interest if the teacher sets the bar too low. Challenge and support go hand in hand. A supportive and challenging learning community, where all students enjoy attainable challenges promotes engagement all round.
Chickering and Gamson (1987) say,
Time plus energy equals learning. There is no substitute for time on task. Learning to use one’s time well is critical for students and for professionals alike. Students need help in learning effective time management. Allocating realistic amounts of time means effective learning for students and effective teaching for staff.
Chickering, A.W. & Gamson, Z.F. (1987) Seven Principles for Good Practice in Undergraduate Education. Online at:http://www.uis.edu/liberalstudies/students/documents/sevenprinciples.pdf