I might have to move because of my sister's college, but I've attended a British school until 4th grade(though I'm not sure that going to help to adjust to 11th grade in a British system).
It think this will be quite difficult because there are 2 types of A levels and normal 11th/12th grade 2 year A-Levels and the 1 year AS-Levels which do not carry as much weight. So you want to get into the A-Levels as soon as possible because they carry more academic weight. So what I'm saying is that the British system has APs that are like AP Calc AB/AP Calc BC which is a 2 year deal or a just AP Calc AB which is a 1 year deal. When you go for your tests, you either take the 2 year deal test or 1 year test. Top colleges generally prefer you get top marks in the 2 year deal which are the A-Levels.
So before you go to the UK and matriculate into a British HS I think it would be prudent to have a serious conversation with the college counselor at the HS there and figure out what your best chances of success are which looks like 3 different paths:
1. Sign up for the A-Levels
2. Sign up for the AS-Levels
3. Take a do-over 10th grade year to catch up with the other students and sign up for the A-Levels.
From what little I know about the British system, I feel that most students there have read more books, are better writers, and have better oratory skills that Americans. Our public school system tend to focus on meeting minimum standards and doing well on standardized testing.
If you want the option to apply to top British/UK schools like Oxford, Cambridge, University of London, St.Andrews, Trinity Dublin, I think you want to have the best A-Levels as possible. If you plan only on applying to US colleges, I think it matters less. While admit rates are not a crazy low as the are in the US, their academic standards are quite high because its more a meritocratic system based on your Grades and A-Level test scores than our "holistic" approach with 100+ data points where ECs, essays and recommendations and whether you are an ALCD make a difference. So OxBridge colleges might be in the 12%-15% range but all the matriculates have high A-Levels and very smart on paper. Oxford is not letting in a B- student on a football scholarship like an Ivy League US school might. The last bit of advice is you want to attend a UK school is to know exactly what your major is going to be prior to applying because in the UK you get admitted for your specific major. You don't get 3 or 4 semesters of college to experiment with prior to deciding. Also College is only 3 years total not 4 years total. So even if you do over a year of HS, you are going to graduate the same age as if you went to American college. So if you are going to pick Math as your Degree in the UK, you will have to take Math exam when you apply to the college. So lets say you apply to Magdalen College Oxford for Math, you have to take a math test with a proctor there. And usually you will be interviewed for a Math spot by a Math professor. If you do not score well on the Math exam or interview, you will not be successful in your admission. Therefore it's more important in the UK to have an academic spike prior to applying to college. You can't just be a wishy washy and say I'm going for Humanities. You have pick your subject like 16th Century English literature or Classics or Law or Medicine. Yes you can apply to Law or Medical school right out of HS in the UK.
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