Yes, 17/18 depending on birth month. There's no leaving voluntarily at any point, would simply be dropping out of school. The final year, 12th grade or Senior year, you finish up like any other year and get a high school diploma at the end. Then you can get a job, join the military, or get a college degree - either by going straight to a four year university you applied to and were accepted for in 11th grade/junior year, or by spending about two years in a junior/community college meeting your general education requirements and transferring to a four year school for your final two, specialized years.
If you drop out of high school, you're severely limiting yourself. Hard to get a job without a hs diploma, etc. You can get an equivalency in the form of a GED, it's a test to show you have the knowledge of someone with a traditional diploma - it fills requirements, but it's also looked down upon.
I'd like to know about the final years of secondary in the UK, I have no idea how any of it works.