[I’m going to use YX/GY here for English and US level equivalents. You’re right that year 10 in the UK is 9th grade or freshman year in the US. I’m also going to use University to refer to undergraduate edu and avoid the word college since it is used very differently in the two systems]
Oooooh that’s a tough situation. I agree with others that Y11/G10 is a really awful time to make that move under the UK structure but family life is more complicated and parents can’t always time these things around schooling. I’m going to do the depressing, here’s the problem post first.
I’m an American expat who moved here for University and naturalized, now married to a Scot, raising 7 DC in England. There are large parts of the English education system I like, but honestly have an intense dislike of the way it structures that stretch from Y9-11/G8-10, and one of the many reasons is that it makes moving schools during those years incredibly prohibitive - in the US it’s a social disruption, but in the UK it can truly be detrimental to a student’s long-term options - and that’s bad for families. As others have said, in the English system students sit their GCSE exams at the end of Y11 and Y10-Y11 are essentially entirely dedicated to preparing them for these exams (Y9 also tends to include GCSE content, but the G8 in the US is more in sync with it). To make this extra “fun” there are multiple exam boards that schools choose from and sometimes options for content areas within individual exam boards so even moving within a specific county in England at that time can be a serious issue.
But the biggest UK/US diversion at Y10/G9 is the way subjects are divided. in the US students continue a broader curriculum for much longer (it still boggles my mine that many UK students don’t study any history or fine art after age 13, or math or any sort of Engish or humanity after 16 for example) but we actually do it in narrower “packages.” So in the vast majority of the US a university-bound student will take a year of Algebra 1, then a year of geometry, then a year of algebra II! and so on. In the UK those three discreet courses are all blended into one maths course over the course of Y9-11, so a student who moves in that stretch may have massive content gaps. In science, US students take one-year courses in each of the three basic sciences, so G9 is often the entire basic biology course, G10 is often all chemistry, and G11 is often all physics. Under the UK system most students will study all three sciences Y9-11, so a student who moves from the US in 11th is likely to be ahead in one domain and very behind in the others. UK students usually get to choose 0-2 from history, geography and religious studies (the humanities - note people here don’t lump english or lit with humanities) and they’ll do a general “history” or “geography” course over Y10-11 (these are simultaneously general history courses but also extremely patchy because they’re taught to the contents of specific exam boards and schools choose specific topics from a set of options to cover and have students test on). US students will almost all take a year of geography, a year of world history, a year of US history, etc. US students usually have to take several semesters from a selection of fine arts and sport or PE options in G9-12 while UK students might choose one of those “practical subjects” to do at GCSE level in Y10-Y11 but it’s done in a way that would be extremely difficult to cram into one year. It’s all just extremely out of synch content-wise during that stretch and I think most Americans have a hard time really wrapping their minds around how thoroughly those years are constructed around preparing to sit the (very much content-based) GCSEs at the end of Y11. The way schoolwork is done and evaluated (the assignments given and how they’re supposed to be answered) is all aimed at prepping for sitting these exams. Americans lament about “teaching to the test” with standardized testing but this is at a whole different level.
The two 16+ years Y12-13/G11-12 are actually an easier crossover in general IMO because at that point students in the US have broadly covered most subjects to general GCSE standards and the UK system allows students to drop to only a few subjects and starts in on a whole new set of content-based exams. A student coming from the US system wanting to do certain subjects at that stage might have some specific catch-up areas but for the most part that’s manageable. One notable exception I can think of is either physics or whichever science your DD’s hs does junior year, which could frankly just end up off the table as an option at 16+ and limit some university options in the UK.