My DD is only just 4, so will be very young when she strats.
But her school do a fab thing, which is that they put the 20 oldest pupils together for their half days. Then they put the 10 youngest together (i.e. in a group half the size) so they benefit from a tiny class for the first term, before they all merge in January.
I remember my DS1, who was also a summer baby, really benefitted from that first term in a class of just 10