It does really. You shouldn't be afraid to go. You should speak. Your voice is important. There are two main ways to go from this. Both involve making list of dates and times of what you list above. ask to say a few words. then first way is pointed thank certain people for their professionalism. If anyone foolishly asks about anyone else, refer to the list or ask who? Second way is to burn your bridges for example list all the sins against you and say this school would be a much better place if these things didn't happen. you might start with "I'm glad to be going because..." Once you have a card etc then take anything free, glass of lemonade cake or whatever and leave. You could always ask the head if there will be chance to make a speech? If the answer is no then leave at the normal finishing time.