This is the story of too many startups in the edtech space. Slow growth, grateful users, but not enough traction to grow the company through revenue or outside funding.
Very few edtech companies have managed to avoid this fate, and often come down to one or two people keeping the product going as a labor of love. There are a number of reasons for this, including the fact that sales are inherently high-touch and expensive, and that the users (teachers/kids) and payers (administrators) have different perspectives and incentives.
Great article. Congrats on sticking it out so long.