Answer keys are a teaching tool, not a grading shortcut
Three ways to use the key before, during and after the lesson — and why the fastest feedback is often the least useful.
An answer key saves time at the marking table, but its larger value is in planning. If you read it before the lesson, it tells you where the worksheet is likely to break.
Before: find the two hard items
Scan the key and pick the two questions that require the most steps. Plan to model one and let students attempt the other. A worksheet with twenty identical items does not need twenty explanations.
During: compare methods, not just answers
Ask two students with the right answer to explain how they got there. The key confirms the answer; the discussion exposes the method. This is especially useful in regrouping and fraction work, where a correct answer can hide a fragile procedure.
After: mark the pattern, not the score
Instead of writing 17/20, note which item type was missed. Three errors in the same column is a teaching issue; three errors scattered across the page is usually attention or fatigue.
What not to do
Do not hand the key to students at the start of independent practice unless the task is self-checking by design. The point of the sheet is to produce evidence of what a student can do without the answer in view.