They will be divided by their age group.. They won't mix all together.. Only montessori schools offers mixed age grouping but there are many parents that are uncomfortable so most of them will still group their children according to their age(nursery, K1, K2)..
Children are usually sorted into age specific classes, unless they are in a strictly Montessori curriculum centre. Visit a few centres that are nearby your place to see the centre and talk to the teachers to find out more about each specific centre.