Hi rosviette.....welcome to the forums.
Up here in Canada, we have several forms of "Immersion". My kids are in the most typical type. In this case, they attend kindergarten, learning french a little more throughout the year, until by the end of the year they are speaking french perhaps half the time. By the ehnd of grade 1 they speak and read French 100% of the time. By the end of Grade 2 they are about 75% French instruction and 25% reading/writing in English, the same for grade 3, and from grade 4 on it remains between 50 and 80% French instruction.
In the US, I believe you would more likely find Spanish Immersion
Hope this helps