While I cannot give legal advice to a non-client, I can offer the following general comments about education law.
(a) I assume that the child in question was attending a public high school at some point. Schools have a legal duty to "child find" students who may have a disability and be in need of special services. (A very bright child can have a disability that impacts his/her ability to access his/her education, e.g., attending school is an issue.) That duty falls on the schools.
(b) In Washington State, attendance at school (or registering a student as a "home school" student) is "mandatory" from the age of 8 to 18. (There are a few exceptions to this blanket rule, but nothing in the scenario above makes me think that the student fits any of those exceptions.)
(c) This fact pattern crosses over into another area of law - one that deals with foster children and the relationship between foster children/foster parents and the State of Washington. While that area is outside my realm of education law, common sense seems to suggest that foster parents (and the State) have a duty to ensure a foster student attends school. And, if there are barriers to that student attending school, then those issues need to be dealt with.
(d) Truancy laws would apply throughout her high school years, but she is 18 now.
You don't state whether the 18-year old is willing to go back to school, or pursue a GED or vocational training, etc., but not all is lost. There's still time to get help for her, if she wants to accept that help. Is that a possibility? If so, then I think that she has several options available, but would need more information.