Is your dd around age 15 now?
So I don't live in Texas I live in Massachusetts and it is very different in some ways but the high school system is pretty similar the country over (with madly different standards).
High School is 4 years - grades 9, 10, 11, 12. She would most likely be finishing grade 9 if she was in the US.
She would have to complete all the credits for high school to graduate - including math, science, language social studies, volunteer work etc. So she couldn't specialist in 3 subjects as I think A levels would be like but would have to do a variety of stuff
They tend to teach in bands (probably more like the UK system than the Irish system I am used to) so in grade 9 do biology, algebra I or II and the language, english, arts elective. In grade 10 do physics, algebra 11 or geometry, language etc.
Then there are AP (college level) courses and honors courses - depending on whether she would qualify for them.
Public schools vary wildly depending on location. They are often huge. While this is a gross generalisation, the quality of curriculum in Texas would bother me - this is a state which could teach about the civil war without emphasising that it was primarily fought about slavery (states rights is the mantra).
How she would get back into the UK system I don't know.
Oh and football (as in america football) will be the be all and end all of everything in that school - in every school in Texas (another generalisation although even in MA schools that have football teams, it is all about the football jocks).
honestly it could be the best thing she did or it could be horrendous. I definitely wouldn't have her start mid year (which OCt-Feb would imply). She should start when the school year starts (which could be as early as August in Texas).
Only you and she and her dad can decide but I would be really really wary of sending a UK teen into a big public high school anywhere in the US and definitely in Texas and thinking it would all be grand. I would really worry about integration/friends/social stuff/understanding the way the lessons are taught (mostly continual assessment so you cannot skip homework and hope to get an A in the final test - if you do that you will fail).
An alternative would be to have her spend summers with her dad doing summer camps etc.?