How do we scale from here? How to get more customers other than spending on ads?
Most school boards have public information on their constituents and meeting minutes.
If you've built this product it should be trivial for you to manually find potential clients. There's no magical "get clients for free".
If you're not willing or capable of paying for sales/marketting, you'll have to get good at pitching (spending your time on the phone). You could attend local school board meetings to meet the right people (spending your time in person), you could offer existing clients (or future clients) referral incentives for onboarding others (spending or portion of revenue).