An elementary school teacher has a 504 plan because of a "scent" issue. (Scents supposedly cause her migraines). Can she force fellow co-workers to refrain from wearing perfumes, scented body washes, detergents, etc.

A co-worker had one. She asked us politely, but it could not be enforced. She had the right to leave places that bothered her breathing, for instance, offices and meetings. Here is some information on it:http://www.hhs.gov/ocr/civilrights/resources/factsheets/504.pdf

In my kids school there's a lady that has this issue. Because of this a letter was sent to all staff, students and parents, asking that no perfumes or scented soaps be used during school.

Can they enforce it? No. It's just polite to keep this persons issue in mind and comply. Try to put yourself in her shoes.

I am one of those people, I get very nauseated in the presence of air fresheners, cleaners, perfumes, etc.
It can't be enforced that people not use these things, but the gesture to not expose them to it is very much appreciated.

The teacher may have some accommodations due to the American's with disability Act but I doubt the teacher has an actual 504 plan. Ask to see the plan she has filed with her employer. It is the employer who must follow any such plan if it exists.