if ofsted thinks a school is outstanding, either
a - the school is really good
b - the school is really good at blagging
ofsted will ring the school from 12 noon the day before inspection. if they don't get through, they'll turn up at 8 30 am the next day, anyway. small schools get a one-day inspection so the call might come mon-thu. average/large schools get a two day inspection so the call might come mon-wed. schools are unlikely to be called on friday because you might spend all weekend tying up loose ends.
so will your child/ren do better?
if its a great school, they have more opportunities to enjoy learning, have their horizons stretched etc. a 'great' school might not get outstanding from ofsted.
but at gcse, attendance and attitude are more important than ability. parental attitude is important throughout.
so if you care about your child's work, think its right for you to take an interest in their progress, keep in touch with teachers, show your child you want them to do well and show appreciation of their efforts and achievement, give them time, space and encouragement to do their work, they will do better than if you leave it all to the school.