At my church, there is a slight difference between Sunday School and Bible Study. In Sunday School, there is usually a different topic every Sunday. In Bible Study, it's one topic going on for a while, then another topic.
Plus, the format is a little different. In Sunday School, u gotta stay on topic and there's a strict time limit. In Bible Study, u can talk about whatever is on your mind for however long u want (although it's usually an hour).