I'm a teacher, and I didn't sign the agreement when in the UK. I always support the school to get the best for my DD.
I think they are an admission of defeat. They set the bar so low: hey, we cannot assume that you, as an adult would not support the school, so here's a unenforceable bit of paper to sign. They've given up on basic decent behaviour.
If a child is out of order, then is the the time for agreements to be signed because they have broken the school rules.
A corollary, and one that was in my last UK school was the rules for the classroom about infringements, warnings, etc. which were a late intervention in the downward spiral of behaviour. They were posted on every classroom wall. The barrack room lawyers in the classroom would call out if the teacher didn't go through exactly the right sequence of steps before ejecting a kid: Hey, Miss, you've not given me two warning/written my name on the board. You can't send me out." Even if I have told you to fuck off.
The school was unbearable.