Random practice is not the goal
Students hear that mixed practice is good and then jump between unrelated topics. That can make revision feel busy without helping them choose the right answer structure.
The useful version of interleaving asks students to compare related problems. What changed? What stayed the same? Which marking points does this question reward?
Related questions make the mix legible
A related-question set can keep the subject matter close while changing the wording, context or command. The student sees which parts of a complete answer still matter.
That makes the hard part explicit. The student is practising how to read the question and build a complete response.
A useful session plan
Start with one topic the student has revised. Open a question and answer it without notes. Check the marking points, improve the whole answer, then try a related question with a changed context.
End by asking the student to name the marking points they almost missed. That turns mixed practice into a compact revision cue.
What to avoid
Do not mix everything at once. Biology, chemistry, physics and maths can all benefit from mixed practice, but the mix should have a reason. A topic, question type or recurring misconception can give the session a thread.