Albani, I am almost positive that we are speaking about schools who allow two (there are schools that allow more) parallel classes of the same grade. Now in Albania this is the most common thing in the world, but in schools in UK and US (private ones here or of small communities, catchment schools, etc) this is done not only to address the number of children, but some times it is done as a manoeuvre in order to become an "integrating" school, meaning allowing for SEN children (Special Education Needs) or gifted and talented children to be supported across a larger year-group.

There are many reasons behind such decision for such schools in these countries. As I said, in Albanian this is very common and only very small villages schools are one-entry schools, but in other countries this is seen also as an attempt to integrate in the society SEN children, as they: a) may not have a chance to go to a Special school which is not hundreds of miles away from home; and b) integration in the society starts from the school. These schools with some more efforts (employing SEN or two-entry teachers) can accomodate such needs of the community and create an integration climate in it.

Viewed from the point of view of a teacher, the two or three entry schools, even when you do not have so much children as to have two full parallel classes, is done so when you have far too different levels in the same grade, so a) you make smaller classes to be able to work more closely with each of them; b) mix the children of the two classes as per their levels when doing math or language to address their needs at a better level. This is what we do at the school where I teach.

Hope it helps.