Any successful academic transition — whether it's elementary to middle or middle to high — will hinge on a child's comfort level. Help your child understand what their new routine will look like. Maybe visit the new school and walk through it with them (schools typically allow and encourage this). You should also help your child understand what changes come with getting older. For instance, explain that they will be more independent as they move up a grade. They'll need to write down their schedule in their agenda. They'll need to bring home and complete their homework.
It's also important for you to be involved with your child's new school and understand the expectations, so you can help your children succeed.