Riven, obviously school would know about your circumstances and hopefully make arrangements to see you separately for your boys. The tests are also not the teachers tests, they're set by the government!
However, as a teacher of 600+ students a week, and a tutor group, the planner is invaluable in informing parents of what is going on in school, and saving time in tracking down phone numbers, waiting in school, sometimes until gone 1900 to get hold of a parent, (and we do have our own kids as well)and having to discuss behaviour issues/lack of homework etc, which could be dealt with through the medium of the planner.
It is also used in many schools for positive comments as well, and is not, as Juuule puts it a 'school-set hoop (signing the diary in this case) which has no relevance to them.' It is a primary method of communication between home and school and is therefore relevant to teacher, parents and student. It is used for sick notes the day a child returns to school; that they have a dental appointment and will need to miss part of a lesson; a child may have a problem that the parent wishes the teachers to know about, and writes it in the planner. It is far easier to write a note in a planner than send the child in with a sheet of paper that will normally get lost/damaged etc over the course of a day.
It is also a quick method of communication as it goes home each day. A letter will have to be drafted by the tutor/class teacher; approved by the HoY/Head of Department or Area, then by a vice principal, and then eventually get typed and sent. That'll take at least 3 days if not more before it hits the postal system. Planners are expensive, but they work and parents have to do their bit as well, which means looking at them and signing them. You'd be livid if something happened at school and you weren't alerted to it. A planner is a quick and convenient way of achieving that.
I also have to say that the majority of the year 7/8/9 students , especially the boys, need the planners to write their homework in, and as an organisational tool, as they tend to forget their brains somedays, let alone their homework. It also starts those who need it, and believe me, several do, on the path of taking responsibility, in that it is down to them to get their parents to sign the planner.
Who would you class as the client of the school Juuule? The child, whom we are trying to educate and organise, or the parent? If you were a client of a lawyer for example, and they asked you to sign something, would you ignore it? Part of a the job of a form tutor is to check that the planners are signed, and the planners get looked at by HoY and in my school, the Principal to check that this has been done. I fail to see why I should get a bollocking because a parent won't sign a a planner despite my best efforts.
I don't expect you to conform to my idea of parenting Juuule, anymore than I conform to yours, but if there is a home/school contract in existence and signed by you, that you will sign the planner every week, then that is what you have accepted you will do, and you should do it, just as I do with my ds. We do it when he packs his bag on Sunday evening for school on Monday, whilst checking all the homework has been done.