Large caterpillars eat a lot of milkweed. You can save time gathering food if you plant milkweed on school grounds. Sometimes it's hard to anticipate when food is needed--perhaps you notice your food plants are dried out--so it's very handy to just go out of your school to pick milkweed.
Some schools have planted conventional flower gardens with milkweed and nectar plants. This is fine, though it might not supply all the milkweed you need.
One way to grow many milkweed plants would be to stop mowing a strip of grass--possibly along a fence at the edge of school property. Plant milkweed seedlings at 1-foot intervals in one or two rows along that strip. Be sure to mark the strip with visible stakes, and post a sign explaining the purpose, along with "No-mow zone."
Tips for growing common milkweed
Gathering seed is easy but there's a very low % of germination. You need to plant a lot of seeds to get a few plants. Sow seeds in an incubator made from a gallon milk jug.Transplanting large plants from the wild or another garden is difficult because milkweed has a taproot. If you take a large ball of soil along with the plant, replant it immediately, and water every day for 2-3 weeks, you might get up to 50% success--but this is a lot of work!
Shoots come up later in spring than other plants. So, if your milkweed is in a garden maintained by weeding, you need to mark the location of milkweed plants each fall. Otherwise, come spring you will think they didn't survive, and weed that area, or plant something else over them.
Mowing once in mid-summer makes for more tender plants. If you planted milkweed in a strip of grass along a fence, it can be mowed once after grass starts growing, but before milkweed emerges. Then mow it again in mid-summer. By September when you need the milkweed for caterpillars, the regrown milkweed will be the tallest plant in the strip, but the grass of only moderate height. Mowing twice a year keeps the strip somewhat tidy and eliminates rangy weeds like sumac. Of course, you could plant the strip with other prairie plants for nectar and eliminate mowing entirely.
Rabbits eat milkweed shoots in spring. They can decimate your milkweed in some gardens, while in others, they may leave milkweed alone. Keep an eye on this--fencing may be necessary. This is why I recommended planting along an existing fence. You can also put chicken wire cages around each shoot until the plants are several feet high.