I'd put the school starting date, for my son, it's 31st January.
My DD2 started this year, it update that she started school, but didn't update the CCB section that she was, in return, I was getting cheaper after school care rate then I was entitled too... And ended up with a debt. So I'd just make sure next year, you ring and confirm all sections are updated!
I'm pretty sure it's solely for the purpose of the School Kids bonus. I just did it also for DD12 (thanks for the reminder) and put the actual date they start and finish school. It didn't ask about dates for High School though.
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