Enrolling in High School

Much will depend on the specific state and school district - so if you can be specific that will help. Eighteen is not a problem... twenty would be.

Transcripts from previous schools - homeschool test records. Again, depending on the school district and state requirements, they might need to take some placement tests.

Ditto on the transcripts. Possibly her social security card or birth certificate. 17 she'd need a parent or guardian there to sign forms, 18 she could handle it all herself.
Coming from homeschooling is a whole other kettle of fish.

Rules regarding homeschool subject requirements, mandatory testing, whether or not they have to be 'affiliated' with a brick and mortar school, etc, etc, etc, can vary not only by state, but by district. It's a whole mishagoss. (craziness, mess)

If your MC is from a fictional Mich. town then you could go by the rules of any district you choose, but find out the general rules in Mich.

Hope she adjusts well!

In the real world, a girl who only needs one year of high school would prob. just take a GED and then get her diploma. Kids who drop out, then change their minds, often do this. If they lack in a particular skill, say reading or math, they take a quick course at the local community college or high school. Or they get some tutoring.

Enrolling in high school for one year just seems a bit odd to me. I can't recall it ever happening in the almost four decades I was a teacher.

But, if, as you say, she 'needs to' for purposes of your story, then in most states she could do this on her own at age 18. (At which point the administrators of the school would prob. question why she is doing so, so think of a good reason for this.) And at age 17 or younger, she'd need a parent to accompany her. She'd need a birth certificate and proof of residence. (In other words, she'd have to be living in the same town or city where the school is.)

Unless it's a private high school, then as long as she meets the school's qualifications and can pay the tuition, she's good to go.

A student needing to finish two or even three years sounds more plausible to me. Or if she were 15 or 16. And a high school senior can be that young, if she's skipped grades or been home-schooled.

One more thing, home-schooling throws everything I said out the window. She could have covered several years curricula in one year. I've heard of kids as young as thirteen-fourteen doing school work at 11-12th grade levels because their parents were uniquely skilled and qualified and the student was somewhat gifted. I've also taught home-schooled kids (who return to public school) and they run the range from kids who could discuss unified field theory (and its flaws) to kids who could barely read and had no idea the moon was in orbit around the Earth.

To enroll, she would need birth certificate, proof of residence, updated and current vaccinations, any previous school records and/or home school documentation (which most states require now, mine does). Home schooled students in our district are required to test for placement so one year of school could possibly become more depending on credits required. Credit requirement for graduation opens up another entire issue. My cousin dropped out after having a child, went back a year or so later hoping to enroll and finish her "senior year" only to learn she could attend but she would be a sophmore so she just quit.

I agree with johnnysanie. If your character hasn't been in school for five years, he (or she) isn't ready for senior year. The schools want proof of learning or credits. She could go to the school (with proper documentation) but wouldn't be a senior. In my state (NY) you can attend high school until you are 21 (the end of the school year in which you turn 21). At that point, you age out whether you have met graduation requirements or not. GED, special programs for those with special needs (disabilities and English language issues) and Community colleges take over from there.