In BGS they set in maths within the first half term of year 7.
Other subjects are taught in tutor groups in year 7 & 8, then set in year 9.
My experience is one of happy children, any problems being discussed and sorted out quickly and fairly, a huge amount of extra-curricular activities in all areas, a brilliant house system, mainly inspirational teachers and definitely academic excellence.
I have an opinion of BETHS, but it is just that. An opinion. I'm sure you will hear fors and against for both. When looking, think estate agent talk and try and read between the lines. Your gut instinct and what fits your child is the way to go imho