His mid-term plan gives science 20 minutes. Do I say something or let the marks say it?

My 12 year old made his own revision plan for the mid-terms and has asked us, quite firmly, to stop checking on it. Fair enough. He's sticking to it, I'll give him that. The problem is what's in it. Maths and English get an hour each most days. Science gets 20 minutes on two days, because he 'already knows it'. His last science test says otherwise. Part of me wants to sit with him and fix the plan. Part of me thinks a disappointing science mark he caused himself will teach him more than I can. He's 12, the mid-terms don't count for much, and it may be the cheapest lesson he'll ever get. Did you step in at this age, or let them find out?