That's why I floated the question about how she functions IRL. Your additional information indicates that her profile does interfere with her access to opportunities and skills that have value for her in her present and future life. In that case, then I would certainly agree that investigating interventions makes sense.
I would not, however, agree that having WMI/PSI scores above 85 rule out "true" ADHD. I have assessed students whose scores all fell in the Average range, but who still clearly met criteria for ADHD. In particular, high-cognitive individuals are often able to use cognition to compensate for executive functions that are weaker, sufficient to raise scores into the Average range. This does not mean that they are not doing so at significant cost to higher-level thinking, in which they could otherwise engage.
As to improving organization and regulation of attention without using prescription medication, I would start from healthy lifestyle habits (regular and sufficient sleep, exercise, diet), outside time, routines for activities of daily living and studying.
You can also try self-monitoring aids. Many children with attentional difficulties do not have a clear sense of when they are attentive and when they are not. You can help her to learn what it feels like to be focused by helping her identify, in the moment and in retrospect, episodes when she is. One self-monitoring strategy may be to set a periodic silent reminder (vibrate) on her phone (if she has one) to remind her to check her attention (say, every 15 minutes), during a time period when she frequently has a tendency to zone out.
Another strategy that can be helpful is to create artificial time pressure by giving her targets for work to be completed before a timer goes off. The key is to set the interval to a span that is just beyond her natural attention span for a task, and to make the target work something that just fits into that span. So if her attention span is about 10 minutes, and she can realistically complete 5 problems in 10 minutes, set the timer for 12 minutes, and ask her to complete six problems, then complete the rest of the assignment in spurts of similar length, interspersed with brief mental rest periods. (If she wants, or needs to, she can reward herself with a very small treat--for example, my DC may eat one Skittle for every one or two problems completed, or make a cup of tea when two paragraphs have been written.) This is also partly about learning what it feels like to be continuously focused and on-task. As she becomes better at this, you can gradually increase the spans, or the work demands (depending on the nature of the task), or both. If using rewards, you can phase out the rewards by gradually reducing the amount or frequency of reinforcement.
Adolescents are also at a point where it can be quite valuable to introduce them to the technological organizational aids that we rely on as adults. Set up a gmail or similar account and have her schedule all her homework, long-term assignments, and upcoming tests and quizzes into Google Calendar, with reminders. This is another excellent use of the ubiquitous smart phones that teens tend to carry. Make sure her calendar is shared to you, so you can be an additional human reminder initially. As she becomes more fluent in using the calendar and its reminders, you will become more of a backup. She might even benefit from using GoogleDrive or other cloud storage for schoolwork that can easily be produced on a computer. It won't get lost, and some teachers will accept electronically-submitted work.
Remember that this is a long-term project. Start teaching her strategies for self-monitoring and organizational scaffolding now, and by the time she is late in high school, she will probably have developed a decent independent ability to use the tools. I've seen a lot of inattentive/disorganized young adolescents develop into reasonably functional young adults, who are, granted, still clearly ADHD, but have developed effective compensatory strategies. The first year of middle school and the first year of high school are difficult organizationally for almost everyone. I find that children with EF deficits tend to lag by a few years, so that, often, some time in the latter half of high school, it seems to come together well enough that organization is no longer a major obstacle to them. This doesn't happen in isolation, of course; that seems to be a common age at which the years of adult-led scaffolding and remediation finally take hold, probably as a combination of overlearning and slightly delayed frontal lobe maturation.
Oh, and on another note: we started homeschooling mainly for academic reasons, as children needed more advancement than was practical in the b&m schools we had available to us, and one was also markedly asynchronous across academic subjects. The one with characteristics of ADHD was actually functioning well as a very young middle schooler (two years young for grade), but that may partly have been because the content was insufficiently challenging, so diverting mental energy to compensating for executive functions was not impacting performance.
