Miss Kindergarten is in the million-dollar club. So are Lovin Lit, the Moffatt Girls and about a dozen other teacher-entrepreneurs who are spinning reading, math, science and social studies into gold by selling their lesson plans online to fellow teachers around the world.

Despite worries from some educators, such online marketplaces are booming, driven by rising standards and the willingness of teachers to pay out of their own pockets for classroom-tested materials.

"I am so thankful and blessed that it came into my life and that my passion and career can kind of mesh into one," says Miss Kindergarten, aka 32-year-old Hadar Hartstein, of Lake Forest, Calif., who says she has earned more than $1 million in sales over the past six years, enough to take this year and maybe the next few off from her teaching job to be with her newborn daughter.

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Her more than 300 offerings on the popular Teachers Pay Teachers site range from free alphabet flash cards and a $1.50 Popsicle party counting activity to a $120 full-year unit on math and literacy, all of them widely promoted on her blog and social media accounts.

"You definitely have to look at it as another full-time job," she says. "You have to put a lot of effort into it."

Teachers Pay Teachers contends that it hit a milestone last year, when its 80,000 contributors earned more than $100 million, and that at least a dozen have become millionaires since the site launched a decade ago. Other major sites including Teachwise and Teacher's Notebook, and recently such corporate players as Houghton Mifflin Harcourt and Amazon, have launched sites of their own.

But some educators worry the increasing monetizing of lessons will stifle the longstanding practice of teachers freely sharing their ideas. And legal experts question whether teachers actually have ownership of the lessons they are selling.

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For teachers buying the materials, however, it's a major time saver, allowing them to reclaim the nights and weekends otherwise spent starting lessons from scratch, often for no more than the price of their morning coffee.

Ann Arbor, Mich., middle school teacher Samantha Cucu said that when she first started teaching three years ago, she created her own materials but was swayed by colleagues who made a compelling argument: "Why are you reinventing the wheel?"

She has since bought about 120 resources and gotten 132 others from Teachers Pay Teachers' free offerings.

"Sometimes they're super-easy small purchases, like $1.20 here, $2.50 there, and sometimes they're larger. I try not to spend over $15," says Cucu who estimated that her prep time for school plummets from 20 to 30 hours a week to two hours if she can find what she needs online.

"It's huge," says Cucu, who has a year-old baby.

At Teachers Pay Teachers, teachers set their own prices for 2.5 million resources and give a commission to the site. With a $59.95 premium membership, the commission is 15 percent. With a free basic membership, it's 40 percent.

"My first sale was 80 cents. It was the best 80 cents I've made in my entire life!" says Mary Beth Nerone, who has been stocking her online store, Brain Waves Instruction, on the site with writing, poetry and other exercises for three years after budget cuts eliminated her job as a middle-school language arts teacher near Rochester, New York.

"Some people are psyched and say I get Starbucks money each week ... some people get to a space where they can meet or surpass their teaching income," Nerone says. "And some people get to a space beyond their wildest dreams of income."

Teachers trust and support other teachers, says Hanna Hudson, editorial director of the We Are Teachers online bulletin board, and accessing individual resources from the web is less expensive and easier than going to the district for new textbooks or resources.

"Honestly, it's going after the money that teachers are spending out of their own pockets on their classroom," Hudson says.

But Bob Farrace, spokesman for the National Association of Secondary School Principals, says taking "proprietary rights over ideas and lessons" could disrupt the traditional collaborative atmosphere of schools. "You want teachers to collaborate and share ideas freely."

In fact, some school districts have language in their teaching contracts that bar teachers from selling their lesson plans.

Some legal experts argue that the resources teachers produce while working for a school district may actually be the property of the school district.

Mark Bartholomew, who teaches copyright law at the University at Buffalo, says that without explicit contract language, the law looks at factors such as whether products were created within the scope of employment or on a person's own time.

A federal court in New York sided with the Cherry Valley-Springfield school district in 2004 after a suspended teacher tried to claim ownership of tests, quizzes and homework problems left behind in his classroom that he said were prepared outside normal hours. The U.S. Court of Appeals agreed the district owned the materials because the teacher made them for his job.

"I never created anything that I was selling while in the classroom," says "Miss Kindergarten" Hartstein. "As a kindergarten teacher, I rarely even had time to sit down and take a breath."

Hartstein says she never looked at selling lessons as a path to riches but as a way for others to learn and benefit from her experience.

"If a teacher is already making a resource for her students and it's super successful in her classroom, then why not post it for other students to use? And if you make something off of it, awesome."