District of Columbia Open Data
Contains D.C. government public datasets, including crime, GIS, financial data, and so on
US school districts and schools by address, with enrollment and boundaries.
DistrictAPI is a free Government API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
REST API for US public school district and school data. Address-to-district lookup, enrollment, demographics, boundaries. Free tier available.
Details from districtapi.dev.

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Contains D.C. government public datasets, including crime, GIS, financial data, and so on
US government data as JSON: SAM.gov contract opportunities, USAspending awards, Grants.gov grants
This API provides read access to DIP entities (e.g. activities, persons, printed material)
The DistrictAPI is provided by districtapi.dev (districtapi.dev). REST API for US public school district and school data. Address-to-district lookup, enrollment, demographics, boundaries. Free tier available.
Yes. The DistrictAPI is listed in the Government category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The DistrictAPI uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The DistrictAPI supports CORS, so front-end JavaScript can call it directly.
Yes. The DistrictAPI is served over HTTPS, so it's safe to use in production.