Correct me if I'm wrong, but doesn't school start when a kid is 6 years old in many parts of the US? If so, that might be part of the reason for the difference in school leaving age. For example, in the UK, schooling is only compulsory until the age of 16, but you start when you're 4. For higher (non-compulsory) education, the common path is two more years at a sixth form college, then three years at University.