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You don't say whether there is any rule, constitution or Articles applicable to the school that would make a vote of confidence a competent mode of proceeding, rather than standard grievance procedure under employment law as would be the usual way of proceeding and as already suggested by the governors. However, on the basis that this is possible the wording would be very simple and would be in the form of a letter addressed to the Board along the lines of "We the undersigned, all teachers duly qualified of xx school, hereby propose that xx, headteacher of the said xx school is unfit to hold that position for the reasons herein stated." It would then be signed and an appendix would have to outline the various reasons for the motion. This is all I can advise at this stage without access to constitutions, rules etc.
I think as many official grievances as possible supplemented by an open letter to the Board signed by as many of the teachers as you are able to get. I agree that strength in numbers is important. The Board are obviously feeling the pain already if the chairperson has already resigned over this. Happy to discuss.
If they are affected by the behaviour of this individual then yes they are important.
Gardening leave is generally where employment is terminated and the person is not asked to work their notice. The term where there is an ongoing investigation is generally known as suspension. Whether or not the person is suspended pending an investigation depends on the nature of the allegations and the circumstances of the individual case.
That would generally merit suspension pending enquiry.
I think it would add weight.