Socks-
Below is a thread loaded with great ideas. I have made natural markers from birch twigs before, slicing off a portion lengthwise to reveal a nice flat, light-colored plane to write on with a sharpie. I like the idea of something natural like wood, copper, or clay rather than plastic in a natives garden.
Ginger

Here is a link that might be useful: Home-made Garden Markers

Thanks. That's very helpful.

Don't use a sharpie to write on plant tags, they fade over time. Pencil lasts the longest on my plant tags.

I went to a local nursery and bought 6" long x 5/8" wide white tags for 5 cent a piece and write on them with a leaded pencil. This should be affordable for your school.

Having been involved with many such projects, I can tell you that having labels done professionally is best. If it does cost more just raise the money. We have asked the local garden club to help with sponsoring such items successfully. Also, parents don't mind ponying up five bucks to sponsor one label. Don't be shy about asking for money from local service clubs like Zonta, Lions, Jaycees, etc.. They all love kids. Have an open garden on a nice summer day and put out a cash donation jar. Ask your local garden centers if they will donate tags. Tree nurseries buy huge quantities of copper tree tags, maybe they will give you a couple of dozen. Good Luck.