I hate to criticize, but I don't consider the Balckberry Lily or Daylilies wildflowers. Both are garden plants from other continents, and not a part of a prairie. If you are hoping to have a prairie, then i'd try to figure out a couple of things:

Who will maintain it and how will it be maintained? if it will be mowed at random times throughout the summer, it may be tough to grow wildflowers and have them bloom. They might grow if repeatedly mowed, but they might never bloom, which makes your effort pretty frustrating. on the other hand, if it will never be mown it will eventually grow brush and trees, although this might take a few years. Ideally, it would be mown about once a year in very early spring to keep woody stuff out but let the prairie plants stand through the winter and provide interesting seed pods and habitat for the kids to enjoy. An occasional burn in spring would be great if the surroundings are suitable.

Can you plant there? if so, I would consider seeds of a few fast growing annual things, but probably seedlings of most plants. Many prairie plants spread nicely from seedlings, so just a few can create a nice stand. Seeds are easy in raw soil, but take time to establish themselves, and don't work well once other plants are already growing. I'd start seedlings, get them nicely growing, then plug them in here and there. This might take many years, but won't be a whole lot of work all at once.

Check out the Prairie School Project. Perhaps you could do something similar in your new school.

Here is a link that might be useful: Prairie School Project

never will be mowed---the slope of the "bank" will make it almost impossible to do. I found my IA native wildflower book last night, oops, purple coneflower is not in there, but lilies are, tiger lilies.

There are numerous species of lilies native to the US, but neither the Blackberry Lily nor daylilies are among them. Daylilies are not even in the same genus as true lilies. I am not sure how well the Balckberry lilies will grow in a poor, dry soil, but I'd be worried that daylilies would take over, even though they prefer moister, richer soils. Anyhow, your book surely lists a lot of native prairie plants that you could try. I'd start with the common, widespread ones or any that are particularly common in the area where you live. You'll want native grasses, too. I think almost any slope could be cut with a handheld weedeater in the early spring to help control trees and shrubs, or simply remove invading tree seedlings by hand (cut, pull, etc.).

Purple coneflower is native to North America, and I would have thought native to Iowa. At any rate, it is common in prairie restoration, and I would feel free to include it in your project.