Well, my daughter isn't in school yet, but my younger brothers and I went to a Spanish immersion school from K-12 and we did half day English half day Spanish from the beginning. I don't think anyone's English language skills stalled at all, like you were saying children get plenty of exposure to language outside of school. We did all our math in Spanish as well and that was fine, I don't think I met anyone in the program who struggled with math because of language. I will say that at least in my experience the program had quite varying levels of success with the students. Some of my former classmates are quite proficient in Spanish and have used it in careers while others don't use it very often. I haven't kept it up as much since I'm a SAHM now, but it does come back to me really quickly when I'm around it. I'm not planning on sending my DD to one of these programs, but that's mainly because Waldorf learning jives better with our family life and the language programs in our area are very non-Waldorfy. Overall I think these type of programs are worthwhile and good for getting the language pathways in little brains set early and I think the families who had the most success with it were the ones who tried to incorporate Spanish into their lives (ie the parents learned the language as well, going to Spanish-speaking countries, interacting with Spanish-speaking people, when she gets older perhaps starting some volunteer work with Spanish-speaking populations, etc). HTH
Stay-at-home Mama to my fabulous DD (10/08) and DS (9/12) and wife to my just-as-fabulous DH
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.
My kids are in grade 2 and 4 of a French Immersion school, they started in Kindergarten, which was 100% French. 10% English was added in grade 2.
We speak English at home. I do know some French but am not completely fluent. Enough to understand their homework though. For us it has been an excellent experience. My ds had some trouble with reading in grades 1 & 2, but really improved in grade 3 and is now at grade level.
Their French is amazing. Both my kids are bright and I think they actually would have been bored in an English only school.
I don't think their English skills have been affected long term at all. They were probably a bit behind their English peers in grades 1 and 2 in writing/spelling, simply because they were not taught any English until grade 2. I am sure they will catch up as the years progress.
We speak English at home and DD's school is in the native language here. We've been living here over 2 months now (we stayed with the in-laws who do not speak English for most of that time) and what I've seen is that DD's language skills in English have really taken off. It's almost like now that she's had much more exposure to her second language she has so much more to tell me in English?
Also if you're a writing major I doubt she's not going to have enough exposure to English! I'm a total book worm (as is DD) and we spend significant amounts of time per day curled up with a good book, I'm not too worried about her English reading skills when that time comes. Even though I speak the local language, at home I only read English books to her and we only speak in English (when we go to bookstores/the library I'll read books in the local language since their English offerings are pretty weak!).
Honestly, since your child will grow up in an English speaking country their going to learn English so adding in Spanish can only help in the long run!
That being said I'd make sure the school is a good fit beyond just the language immersion thing (as in they have good teachers, you feel comfortable with the class sizes etc).