Add up the market sizes for: private schools, private tutoring (Kaplan, 1-on-1, etc.), test prep (SAT study guides, etc.), for-profit colleges (Phoenix), language learning (Rosetta Stone), what teachers spend in-budget and out-of-pocket for their classrooms, education iOS apps, etc.

I'm not saying Quizlet does all the same things, but nothing about paying for extra education is what I would call niche.