What is challenge?
The idea of embedding challenge in lessons is to get students to think and stretch themselves, it is not to create more work or get staff to rewrite schemes of work. In fact ideally departments should adhere to schemes of work and simply change the way in which they teach them. An example (and I apologise to History in advance) if you were teaching causes of WW1 to a class rather than deviate from the scheme of work get them to discover how a single bullet (the assassination of Ferdinand) caused 15 millions to die? They could be given a series of clues (differentiated) that piece together either in groups or as a class the puzzle.
Why is challenge important?
Challenge is important to stretch and challenge all learners.