"Revolutionary." "Disruptive." These terms are used with such frequency that they may have lost much of their meaning. That's not to say that there aren't plenty of products and services that are innovative, and plenty of systems, plenty of organizations that are ripe for disruption or "revolution." Take education, for example. Our modern education system is, after all, not so modern, with many of its practices strongly rooted in a "factory" model circa the Industrial Revolution. But what does revolutionizing education really look like? And which startups working in education technology are really "disruptive"?

A recent thread on Quora bypasses the "revolutionary" and "disruptive" adjectives, asking instead "What are some interesting startups in the education space?" But a recent blog post at The Teaching Master does invoke these adjective, listing the "Top 25 Web Startups Revolutionizing Teaching." Neither the Quora nor the Teaching Master post offer metrics. There's no indication of what makes a "top" startup or what constitutes "interesting," let alone "revolutionary" work in the ed-tech space.

Is it based on profitability? Scale? Test scores? Overthrowing old theory and pedagogy? Overthrowing established companies in the industry? And as one of the companies on the Teaching Mater list was founded in 1999, I feel I have to ask again, what's the definition of "startup"? (And I should note here that this doesn't mean established tech companies are not doing innovative things or that educators aren't using older tools in the classroom in innovative ways.)

In preparing to write several back-to-school stories for ReadWriteWeb, I initially considered a post featuring some of these "interesting" and "revolutionary" ed-tech startups. But instead of another list of disruptive ed-tech startups, I'd like to offer a list of the 5 disruptive things ed-tech startups are doing. I would love to hear your thoughts - as educators, as entrepreneurs - on what other metrics you might use for evaluating this disruption. After all, if as Bill Gates suggests that in five years the best education will come from the web, we should probably consider now what we want that education to look like if it's to be different than just an online version of that old factory model.

1. It should be free(mium)

To say that schools face budgetary restrictions is an understatement. Yet many tools - hardware and software - aimed at the "enterprise" education market require a sizable investment. "Free" is the right price for many schools and teachers. It's not that schools can't or won't pay. Many can, and many will. But the cost should not be such that schools must choose between implementing a new technology and axing the art department.

2. It should encourage grassroots adoption

Along with the right price comes the right marketing and adoption strategy. As such, many disruptive education technologies are aimed at the individual teachers and students themselves, rather than at the districts-as-a-whole. This is important as this grassroots approach means that the tools pass the "smell test" of teachers in the classroom, meaning that the tools are usable and useful. With a multitude of free tools to chose from, however, interoperability will be key so that educators don't find themselves locked in to one product or service.

3. It should encourage 21st century teaching and learning

The teacher once stood at the head of the classroom, the focus of attention, the holder of knowledge. Students worked individually at their desks. 21st century teaching and learning challenges this by de-centering the classroom, by emphasizing collaboration, by encouraging social learning. "What kids should know" has expanded beyond just core subjects to a blend of skills, content knowledge, expertise, and literacies. 21st century teaching and learning is not restricted to classrooms and textbooks. It is online, and it is mobile.

4. It should contain and encourage open content

Digital creation and collaboration may be challenging the ways in which we think about intellectual property in the classroom. Alongside budgetary concerns, many educators are embracing open content - OpenCourseWare, open source textbooks, and other Creative Commons licensed materials.

5. The technology should be open source

Open source technologies help address many of the points listed above. They help schools avoid being saddled with restrictive EULAs and licensing. They bring a community of developers to work on a project. Even better, they allow students themselves to dive into the code.

What's missing from this list? What other ways can and are education technology startups disrupting education? And what startups are doing so?