If you are saying that governors' meetings should be conducted in an orderly way, sticking in a broad sense to the agenda, not bringing up any major new, even if related, issues in and around the subject without prior warning, I would agree with you. I would also counsel parents and staff governors - and indeed often do - that though it isn't written down anywhere, it is both courteous and efficient to inform the head of any new aspects of an agenda item or any very controversial issues they intend to raise at a meeting. That does not mean you have a veto, but simply that it is inconsiderate to put anyone on the spot in a public setting, and isn't likely to get a thought-through and rational response. You do not have a veto and I hope your letter does not arise from a desire to suppress debate. Indeed if you are a bit high-handed with staff and unwilling to discuss difficult matters concerning them within the school, their representatives may well be deliberately trying to trap you into an answer in the meeting. That isn't nice but it is pretty natural.
You do not mention the chair, which is surprising. It is his or her job rather than yours to ensure that members keep to the agenda, ask colleagues' permission to add an item if it has come up suddenly and cannot wait, and rule out of order any interventions which really do threaten the proper conduct of business. Perhaps you should have a discussion with him or her about your concerns. A good chair would not be so hide-bound by the letter of the regulations as to silence any member who brought up new and important matters of real relevance to an agenda item. On the other hand an experienced chair would be pretty smartly on the case if he or she thought someone was taking advantage of the presence of assorted others to cause you discomfort. A really super chair might try to discover why you felt so threatened by unexpected comments by staff, say, and help you to overcome any touch of that paranoia which most of us exhibit in difficult corners.
If you think staff really are deliberately wrong-footing you, talk to their representatives about governors' meetings. Make it clear that you do not want to stop them doing their proper job of representing, but that they will get more out of their role if they tell you in advance of difficult issues they intend to raise, simply to ensure they get a considered answer.
Promise them you will not try to muzzle them.
Questions for Joan Sallis should be sent to The TES, Admiral House, 66-68 East Smithfield, London E1W 1BX, fax 020 7782 3202, or see www.tes.co.ukgovernorsask_the_expert where answers will appear