Bililingual education is controversial for two related reasons.
First, there is the issue of whether bilingual education is actually the best thing for students who do not speak English or do not speak it well. There is a great deal of controversy over whether that sort of education speeds such students' academic progress or whether it actually retards it by slowing the students' acquisition of English.
Second, there is the political/social issue of assimilation of immigrants. Many people feel that it is simply wrong to teach students in America in a language other than English. They feel that it sends the message that it is possible to get along in the US without speaking English. They also feel that it uses government funds to encourage immigrants not to assimilate.
For these reasons, bilingual education tends to be controversial both among educators and in the broader society as a whole.