petitefrancaise

Westwood HS is the IB school for that area and is the best high school in Round Rock ISD.There is a charter school (Meridian) in Round Rock that is excellent and does the IB but entry is via a lottery and that's been done already for the next academic year.
Just a few notes:
keep your options open regarding University - if you have been on temp visas in Singapore and are still on temp visas for the USA then you will probably be ok getting domestic UK fees. If you have been on permanent resident visas, probably not. Your kids may well want to go to a US university as well and if they go to an in-state Uni, then the fees will be similar to the UK fees.
As far as I know, UK universities will also accept AP exam results, so you shouldn't rule out doing that. In Westwood, a lot of the IB kids also take the AP exams simply because (in most cases) the courses are very similar.
Depending on where your OH works, getting to Round Rock is very quick and easy from Westwood school boundaries ( straight over on I45 which never has any traffic). Renting a house is tricky though, there is almost no availability. I assume you are getting some help with this?
Other high schools in NW Austin that are reasonably good - Anderson HS. Does IB. McNeil is OKish, Vista Ridge OK. the rest I would avoid.

robtuck

Our daughter is on an L2 and was able to apply for USA Universities with no problems. They push you to convert to F1, but as we were also seeking a Green Card we were advised not to adjust her status and Uni has been fine with it. She is being counted as an International Student for accommodation purposes but received In State Tuition due to her GPA & SAT results. The temp status just meant she was not eligible for the State Scholarship scheme, otherwise it's been pretty much plain sailing. As an International Student they did try and push her to get Medical cover through the Uni, we had to show evidence that she was sufficiently covered by our Employer scheme