This would only work if every teacher did it. Odd ones here and there will have little effect. If the research referred to by @Catgirl1964 is correct, then even if the teacher gets it 100% correct in lessons, it will not contribute much to the end result anyway. The real issue is the reluctance of students and parents, to accept thay actually have any responsibility for final outcomes. The extra time might allow students to catch up on project-based work they should already have done, but I don't think desperate last-minute revision for exams will achieve much.