I agree that 16 is waaay too young to be out of school. It's preposterous to expect a 16-year-old to know what they want to do with the rest of their life.
I came to the UK the first time in 1962, was only 12 years old and had to spend all my time playing catch-up with my pears in school, needless to say I did very badly. I left school at 15 and went on to an apprenticeship because my parents were not educationally oriented and thought so long as you could read words with 2 syllables, add, subtract multiply and divide, this would set you up for life

. Later when I returned to States I had to spend a lot of time getting my education in order as a result of my parents wisdom. Fortunately I was
motivated enough to rectify this and eventually went on to university, albeit much later in life.
With this in mind I have a particular beef with anyone thinking of introducing US kids to the British education system if that child is older than 5 or happens to have an IQ level in excess of 200 and is particularly gifted. First it's hard enough for a child to take on a new culture where the attitudes and beliefs take some getting used to, but add to this the fact they will be at least 3 years behind the class they join, that the teaches will not always be sympathetic to their problems and you've got a recipe for misery for that child.
It's my view a child should be allowed to enjoy their time at school, and parents need to think long and hard before subjecting their children to a major educational change mid-stream. If you believe that extra tuition and hours over and beyond that of the usual school day is a pleasure for a child, I would suggest it's worth another look at. Britsh schools are more demanding then the average US school, the outcome of exams are less forgiving in terms of a childs future and playing catch-up is something that most children will not appreciate.