If all you are interested in is getting your child into a Thai University (rather than a fully rounded education and all that it offers), check out the GED program (This is a US/Canadian high school equivalent qualification). There are a number of tutorial classes/private tutors that could provide tuition, it should be possible to pass within a year no matter what her current grade level, if she works hard.. The GED has just 4 subjects (Math, Science, Social Studies & Language arts) and is accepted by Thai universities.
If you are at least 18 years old, you can apply to take the GED. If you are 17 or 16, you may be able to apply to take the test if you meet additional requirements.