How pathetic ! How damaging ! Don't they have anything else to do ? Were you given adequate notice of this or was this spontaneous ? Presumably you and most of your colleagues have been given gained time tasks to do which are essential to the running of the school especially as SOWS are being revised for the new curriculum. Yes SLT /HT can and will do as they please especially if no one stands up to them. It is essential that you and your colleagues call for a union meeting and put your cards on the table ...list everything why this should not be carried out , what will it result in and what exactly are the benefits of doing this so late in the term. Smells like some sort of secret agenda rather than genuine school improvement strategy. If you are secondary this is even more pathetic as half the school is out. Clearly your SLT have no idea on staff retention strategies and need to change their policy on staff observations.