In my own situation, I’d take the class of 35 or choose a school and drive ten minutes. My dd is oldest in her class and able, so to feel she was doing y5 “again” would have made her feel very unhappy and isolated. She was bored in her own year group, repeating work that many kids were struggling to understand after missing out from Covid, so I imagine being with the kids going over last year’s curriculum would be awful.
Totally different if the school planned a mixed year class - that can work brilliantly. I had two of those at my small junior school, but there was no national curriculum and the teachers did it every year, they taught us to work independently or in small groups. We had no whole class teaching of maths, so I was never left staring out of the window waiting for time to pass like my poor dd has been. Imagine having to sit listening to what y4 is doing - all your friends - waiting to be given your own special extension work for Y5. I cannot imagine even in a small class, a teacher would manage that particularly well (lots of wasted time for your niece).
theres a chance it will only be a class of 33 if the other two applicants choose to go elsewhere.
I would NOT go into the year below. 18 kids doing Y4 work, and just one or two doing Y5? No thanks. How would your dsis feel if the other two parents chose the larger Y5 class and yours was the only one left behind redoing Y4 (because in reality, that is what will feel like is happening )? What would happen next year - how can a teacher prepare one single child for SATS?
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