I am looking for some guidance with High School, we have a daughter who is currently Year 10 and looking to relocate to US at some point this year (hopefully before the next School year). Do I contact the High Schools in the location we are looking to move to directly? I realise it is quite a vague question I just don't really know where to start.
Thank you.

Usually there’s no choice of high school, since most states are divided into school districts, and depending on your residential address, you attend the high school in that district. (Sometimes there’ll be a choice, such as between an academic high school and a voc-tech high school.)

So, one strategy is to research high schools, find one your child would be happy in, and then look at housing options in that exact school district. This obviously presupposes you know approximately where you wish to live.

So, two things:
1. You may want to wait to move until your child finishes secondary school. Moving a child to the US halfway through high school is often hugely disruptive, partly for obvious reasons, and partly because the US system runs a system that gathers credits for each course throughout high school, and getting coursework in the UK accepted for high school credit can often be very difficult or impossible. Some people suggest finding a high school that offers international baccalaureate (IB) to help with transferring, but most do not.
2. Actually, Robin is incorrect, there often is a choice of high school, but that just highlights that there is a lot of variation in the education system between states and even between school districts. Without knowing where you're looking to move to, helping you is difficult.

Agree. The US system is designed around a four year HS program (grades 9 thru 12). You build credits toward graduation in all of those years. Grades 11,12 are particularly important for college prep with options to take Advanced Placement (AP) classes for college credit if your child is ambitious but it's all built on starting in grade 9. Some areas have private schools which gives you an option outside the publicly funded system. But as previous posters have said, an international move in grade 10/11 will be very disruptive. A good set of UK A/AS levels can be used for applications to US universities so delaying until after HS graduation would be best if possible.

We moved to US June 2018. My 14 yr old son started high school (grade 9) August 2018, it was tough!! I don't advise it. Took a while to settle and a year or 2 to figure out that every single grade has to be consistently high from the get go as GPA is everything here. very different to UK. He is at college now but it was much more difficult for him than it was for my daughter who got a year of middle school in first - which gave her some high school credits (can get these in middle school, takes time to learn the school system here!) and a head start.
* ps I did have a choice somewhat as to which high school, as my son when to a public high school and my daughter got accepted (via lottery) to a charter high school

one more point tho that I just thought og=f, I don't know about other states, but Florida offers an amazing scholarship which covers all tuition for Florida public universities if you meet the requirements and have been a Florida resident for (I think) 12 months before applying for college. This was a huge benefit for us so worth looking into when weighing up pros and cons of UK vs US high schools

This is a good point regarding state college systems. If you wait until after A levels then move here, state college systems have residency requirements to get the preferential 'in state' tuition rate (each state funds its own college system from state tax revenue for the benefit (mostly) of its own residents). I don't know if they waive those requirements for new immigrants - each state does its own thing. The disadvantage of not qualifying for 'in state' rates is you are charged the 'out of state' rate which can be *much* higher.
There are of course private universities with no in state/out of state quotas or tuition rate differential. Full fare at those is expensive but all offer scholarships which can be surprisingly generous.
All this assuming your child might be college bound.... in which case you coming here and sending your kid to uni in the UK is also an option.

even tho my kids had completed 4 years of high school in Florida, we had to jump thru hoops and it literally took months to prove their Florida residency for college. every time I provided what they asked for, they requested something else... payslips, tax returns, every driving license from both parents (had to renew annually while on visa), it seemed never ending. I would not like to go thru that as a new immigrant it was difficult enough after obtaining green cards and 4/5 years living here!! I guess they have so much fraud to obtain in state tuition rates....

Thank you for your replies. It does seem to be minefield due to my daughter's age, we are still looking into every option for her at the moment, but wanted to just get an initial understanding of what it would involve at her age.