CREATE TABLE classroom_muted (
  session_id INTEGER NOT NULL REFERENCES classroom_sessions(id) ON DELETE CASCADE,
  user_id    INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
  muted_by   INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
  muted_at   TEXT    NOT NULL DEFAULT (datetime('now')),
  PRIMARY KEY (session_id, user_id)
);