$28k per student is more than enough to run a school in San Francisco. Let's assume we cannot take advantage of the economies of scale available to SFUSD, and we're running a school with just one classroom: 22 7th graders. That would cost SFUSD $616k ($28k x 22). What would it cost us?
Teacher (all-in cost): $150k
Teaching assistant: $100k
Rent for commercial space in SF (~1,200 sq ft): $60k
Curriculum, books, supplies: $23k
Technology (22 Chromebooks, projector, software): $18k
Field trips and enrichment: $10k
Utilities, internet, insurance: $27k
Furniture and equipment: $20k
Admin/legal/accounting: $8k
Total: $416k
That leaves $200k unspent.
AND ... these numbers are deliberately conservative. Teachers work ~40 weeks per year, not 52, so the $150k all-in is really $3,750/week - very competitive for SF. The $18k technology budget assumes replacing every Chromebook annually, but they last 3-5 years, so amortized cost is more like $5k/year. The rent estimate of $5k/month assumes market-rate commercial space, but you could find cheaper options in underutilized buildings or negotiate with a church/community center. Furniture lasts decades, not one year. The $1k per student for curriculum and supplies is also high - you're not buying new textbooks every year, and open-source curricula exist.
If you were trying to minimize costs rather than be conservative, you could probably run this one room school house for $350k/year ($16k/student/year).