Just want to add my two cents. We don't have end of the school year parties, at least not then...However, for the past several years we have had OPEN POOL DAY at our house. ( I think this could easily be done at a community pool too)
Our now 7th grade son has loved this all summer long invitation for his classmates to come and swim. It helps keep him from thinking that he never sees his friends. We send out postcards at the end of the year letting everyone in his class know when we will have OPEN POOL. It can be once a week for a month or whatever will work into your summer. We sometimes call and remind his closer friends before the dates and enjoy setting up a fun day for "whoever" shows up. There have been some times that no one came. We have a back-up fun project or outing "waiting in the wings" so that either way he is looking forward to the day.
Along with my summer "school" system it has worked really well. It is also usually smaller groups and parents that spend the 2-4 hours with us and friendships develop with more ease.
Enjoy your summer too.