Many times, it is better to allow friends and their parents to get to know your child before telling them. That way, they know your child as a person before they can stereotype him. This, too is a judgement call. Generally it is a good idea to let the school know if your child has any special needs. However, this can be a particularly thorny issue if your child is applying to a competitive private school.
Some schools are more understanding about ADHD than others.
If 100 children are applying for ten slots, a few schools might not spend the time to understand your child's unique situation. Talk to other parents and get a feel for how the school personnel deal with these issues. If you know the parent of an ADHD child who attends the school, they might be able to give you advice. If your child is currently attending a particular school, one should be sure to tell the school nurse about any medications, even if they are given at home. Children do have accidents at school and the information should be available for emergencies.