My DS is in a trilingual school. He had full Spanish Immersion in K and 1st (no English at all.) In second he started getting the percentages like you mentioned. As a 5th grader, he's about 50/50. He started getting Mandarin Enrichment 4 days a week in 3rd grade.
It's been a fantastic experience. DH speaks a little Spanish. I don't speak at all. We haven't had any issues. DS's English is excellent. In fact, he's 10 and his recent assessments put him in the high school level for English reading and comprehension. The school's state test scores are excellent. In fact, immersion schools usually follow this pattern when it comes to testing... 2nd grade the kids test a little lower than their English only peers, 3rd grade they are about the same, 4th and 5th grade they tend to test about 30 percent higher and that is testing in English, not in Spanish.
I know that some of the parents complain that their child's English spelling is poor but they seem to adapt quick once the expectation to write in English is there. Personally, we didn't have the spelling issues. DS's understanding of language and how it's constructed is pretty amazing... much stronger than my eldest child who went to an English-only school.
As a parent, it is important to continue reading in English to your child but I suspect you'd do that anyway.
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