No I don't think so. I never had problems with retention over the summer. Keep summers free, especially for students old enough to work. You can learn a lot from your first few summer jobs that you can't learn in school.
Our problem with schooling in the U.S. is not summer vacation. It's blood sucking teacher's unions making it damn near impossible to fire bad teachers (sure the Unions have their benefits but on the whole they are corrupt) and legislation such as no child left behind (more like, "smart children dumbed down").
In short, fix bad teaching and bad legislation instead.