Stretching high-attaining pupils does not mean giving them more of the same work. The most able pupils are often under-challenged in mainstream classrooms — they finish early, then either coast, or do extension work that feels like punishment for being quick.
Effective stretch comes through depth (going deeper into the topic), complexity (introducing more variables or nuance), and abstraction (moving from concrete examples to general principles). 'Why does this work?', 'When would this fail?', 'What if we changed this assumption?' are far more powerful than 'now do question 11'.
