Some children love to challenge each other to be the first to hand in the paper. If they have a lot of time still before the time is up, submitting the paper early reaps no rewards.
Instead, he should be taught to use all excess time to check, and double-check through his working and answers.
Now what if there's not much time left? Should the child attempt the rest of the questions, or check through those that he had done?
Personally, I would encourage my boy in invest the last few minutes to ensure that all the pages are filled, and all questions attempted. If he missed any questions, at least he has the chance to attempt them. In essence, I think it is better to write down some workings than to leave a question completely blank.
Experiment #7 - Forces (Frictional Force)
Experiment #6 - Forces (Frictional Force)