Call off every class on the chosen days, one action, one week.

The same write as "call off this day" (organization_routes.cancel_classes), repeated per day: scheduled rooms only (a running class is never ended from a calendar, an ended one is history), projected classes built in order to be marked, and each day's students told through the existing notice. No announcement is posted from here; the screen offers one, as the day dialog does. Undo is the existing per-day "put back".

Whole school needs a coordinator (owner or admin); group_id narrows it to one section, which the section's own staff may do.