Practice separating by having your child stay overnight with a family member or go to a play date without you.
Before school begins, start following what will be your school routine. Have your child go to bed and get up at the time they will for school, set out clothes the night before, get up and eat breakfast, etc.
Role play, practicing the first day of school – you could even play the part of your child and your child the parent. Practice things you might do or experience together.