Thanks for directing me here, Strudelface.
We moved a year ago when DS was 4yo, he went straight into 1E (premiere enfantine). 1E and 2E are not compulsory but almost all children do go to school. Here in Geneva we don't have kindergarten, the 4-5 yo go straight into schools.
DS can write his name, the names of the family members, ie Mama Papa and his sister's name. He is learning joined up writing. They also do some maths, but very basic stuff. Counting to 20, practicing writing numbers etc.
I asked the teacher last year and she said that they have one activity that they have to do in the morning, once they have completed that they can choose what to play with.
They do a lot in our school, trips to theaters, museums etc and often work these things into their lessons.
Recently DS has been doing a project on Boucle d'or et les trois ors, Goldilocks and the three bears. He brought home a folder of work that he had done.
It is not really comparable with nursery, there is more formal learning.
Saying that, DD did not have any of that and moved here from Germany where they don't start school until 6yo. She has not had any problems (aside from the challenge of learning French, which has gone well).
I am a great proponent of local schools. If your children were older I could see the point of sending them to the IS but at that age, your DS will really benefit from the local school. And he will learn French very fast. DS is very fluent, and translates for me now.
If you live within a certain distance of the French/Swiss border and you or your DH work in Switzerland, then your DC can choose to go to either French or Swiss schools.