My first paid assignment was writing test questions (in industry lingo these are called "assessment items" or just "items"). With that experience under my belt, I was hired on short notice to write tests for another company. That editor liked my work and gave me more assignments, mostly editing and writing the workbooks that accompany textbooks (known as "ancillaries"). And soon after I began landing assignments to write children's books.
What is the best advice I ever received about how to succeed in education writing or any type of freelance writing? It is simply this: concentrate on pleasing your current client. Do the best job you can, even on the most mundane assignments. Deliver well-written and carefully edited copy that meets the editor's specifications, be polite, don't whine, and turn everything in on time.
If you are interested in writing for the education market, here are a few articles you may find helpful:
"Writing for a Book Packager" by Rusty Fischer
"Writing for Curriculum and School Library Markets" by Rita Milios
"Packagers" by Ben White
"Above and Beyond" by Jenna Glatzer
And here are some good places to look for freelance opportunities:
Get curriculum development jobs
Writing for the Education Market
My favorite children's writing spots
Finally, there is a wealth of information on the internet about writing for children. Here are some of my favorite sites.