Just wanted to chime in...as someone who grew up bilingual (speaking both in school) and who now lives in Poland where her children attend American/International schools, I can see both sides of this issue.

At my son's school, the language of instruction and communication is English.  The school is a cultural mix of American, Polish, European (all countries), Korean, Japanese, you name it.  The children know that they must communicate in English during school hours.  The main reason for this rule is that without it, children become cliquish, sticking with their own culture instead of mingling with everyone. And let's face it, if you are in a group of Polish kids and they are speaking in Polish and you do not know the language, it is pretty rude.  Overall, the kids that attend this school are fluent in English as a second language, so their ability to speak the language is not the issue.  Further, if they are new and have little English language ability, the teachers and classmates work with them.  I have yet to see these kids go over two months without the ability to communicate in English.  They are sponges.

Kids do get reprimanded for behavior irregardless of the language used, but they are also reminded to communicate with their peers in English.  I agree that behavior is to be reprimanded first and foremost. At the same time, if the common language is English (or Polish or Spanish) then I think that kids need to be reminded (not reprimanded) to speak the common language. Otherwise, there is an unspoken segregation occurring between different ethnic/cultural groups.
