Regenerate Class Report

Regenerate all per-student feedback cards + grammar/vocab for this class (main room + every breakout), then the class report reflects it on next open.

Teachers use this after editing their teaching philosophy so the feedback is re-written in their style — or after a bad generation. Heavy (several Gemini calls per student per room), so it runs as a background task; the client should re-fetch the report after a minute or two.