It's hard for me to answer such questions without knowing all the family circumstances and the language abilities of the kids, so I ask parents to consider the following factors in making their decision.
Location: One of the most important considerations in deciding on which school your child will attend is where you live, where the school is and what methods of transport you have at your disposal. The school you have chosen could be Montessori, have Chinese language immersion and serve organic lunches. If it's across the city, then it won't be much use to you unless you own a helicopter.
Is there any connection with your family or with where you are living?: As an expatriate, would I ever send my children to the school where the Spanish kids go? Not unless we were expatriates in Spain or another Spanish-speaking country. I'm not Spanish or even Hispanic and neither is my partner. We don't speak the language, don't know the culture and wouldn't be able to offer an iota of help to our kids. There is simply no family connection to the Spanish language or culture. I would rather send my kids to an English school and get them really good in their mother-tongue than put them in a school where they have no connection with home or where they live.
Keep in mind that I am not talking about language immersion schools that are AIMED at non-native speakers, such as French immersion schools in Canada. I am talking about sending your kids to the French school where the French kids go or the Spanish school where the Hispanic kids go or the Japanese school where the Japanese kids go. Your child will suffer unless there is some kind of, well, reason to send them there, like that you happen to live in France or Spain or Japan, or you used to live there and you don't want your child to lose the language that they have learned, or that you are French or Spanish or Japanese.
If your family is bilingual, which school will maintain the weaker language the best? I speak only English with the kids and my partner speaks only French with them. The Bambina is in the French system because, without the support of school, French would be her weaker language. Papa is simply not home enough to ensure that she is is getting sufficient exposure to French.
Considering choosing a local school if you speak another language at home and if you are going to be living in the country for a long time. Why not? You live there. Your child might as well learn the language of the street.
Language immersion schools work best when there is support at home. If you are just a North American family living in North America and want to put your child in a French or Spanish immersion school that is aimed at non-native speakers, consider how much support you will be able to offer your child. My experience with language immersion kids is that, without the support of a family member who speaks the language, these children end up not being able to speak the immersion language well and, worse, not being able to read or write their native language well. In the absence of a good support system at home, I would choose a plain old good quality traditional school over a language immersion school.
