My DS is 4 years 4 months and has recently started at a state primary school with a large reception class of 45, divided into two classes for academic work but otherwise mixed. He is happy at school but I've noticed when I drop him off he turns into a child that is paralysed by shyness. He seems incapable of doing anything and just stands there looking lost. 
Prior to this he was at a small private nursery in a class of 10 children, in which he thrived. At home he is a very talkative, bright, and confident child who has a lovely personality and dotes on his brother who has SN.
I am so concerned about why he is like this at school. Not sure whether it's a case of settling in or whether the school is completely wrong for him/too much of a change from the size of the nursery, and he really needs smaller classes where he can feel part of something again.