Ask your Head Teacher at what time the meeting is scheduled to finish under the agreed directed time schedule.
You are entitled to leave the meeting at the scheduled finish time, regardless of whether or not the meeting is still going on.
It is often sensible to encourage meetings to start promptly, so that trapped time does not account for wasted time.
Your school should have consulted on and produced by September 1st a directed time calendar which specifies exactly how your 1265 directed time hours are to be allocated.
However, pragmatism is always a sensible approach, as some meetings may be considerably under time allocation.