The structure of language immersion programs differ a lot depending on the school. Ours is a "dual" immersion program in a public school, so half the children are native Spanish speakers and half are native English. The PreK that my son is in is entirely Spanish. They do no formal literacy instruction (just letters and letter sounds in Spanish) and math in Spanish. Then in K it is more like 80% Spanish with about a 45 min. lit block in English for native English speakers so children learn to read in their native language, but all math, science, social studies, etc. is in Spanish in Kinder. Every program is different, so you have to find out what kind of program is available in your area. Some schools are one way immersion--meaning that there are only non-native speakers of the language in the classroom. These are usually private or in non-diverse areas. The disadvantage is that there are no peer models and I would guess that the language complexity is less since all the children are second language learners.
