I'm wondering if it's a texture or sensory issue? The fact that she's not eating at school or throughout the day is very interesting - it's not just as home to get your attention.
I agree with you that you don't make 8 different meals for 8 different kids, but I think it's hard to force a kid to eat things too (and it can create more problems than it solves). At 7, she is capable of doing a lot for herself, so I suppose if she is making herself a PB&J sandwich instead of the dinner you fixed, that's possibly a solution. If she is demanding that you jump up and make it, that's something else again.
There are pretty simple ways to sneak nutrition into many other foods, so that's one way to go if you're doing it for the whole family. I was a master at getting tofu into things so the kids got protein when they didn't think they were, that sort of thing. There are tons of recipes now that add squash or cauliflower to the mac & cheese, for example. Same goes for healthy fats. Most kids like "breakfast foods" as yours does, but if that's all cereal and English muffins and no protein or fats (which will keep them satisfied longer), it's a problem.
Have the teachers commented that she's falling asleep or acting cranky, or do you find her lunches coming home untouched? Are there other examples of her having any sensory issues, or is it just around food?
Making food a big issue can create problems, but you can't ignore it if she's really depriving her brain of vital nutrition. If there are no major warning signs, I'd start by limiting the amount of filler foods she has access to, and letting her make her own substitute meal if she doesn't like what's offered. That also means cleaning up her separate dishes and cleaning up the prep space. If she gets tired of the work, she may decide it's easier to eat what's there. Where possible, if you can let kids assemble their own meals (through a giant salad bar or taco bar) that is the same amount of prep for you (chopping veggies, for example, and just putting them in separate bowls, that might give her a certain amount of control. If that doesn't work, and if the problem continues, you may have to have her evaluated.