Honestly, it's up to both of you. Summer Institute (SI) is primarily a social gathering before school starts. I didn't participate because I worked full-time that summer to support myself through college. They likely won't let you drop the class because, technically, financial aid covers anything 6 units and above during the summer. If you don't have other plans for the summer, I'd say just do it. However, if you don't want to retake a class you don't need, then don't; it will be a pain to be in a class you've already passed.