ELLs and the law
In 1974, Lau v Nichols stated that treating non-English speaking students and English speaking students as equals in public schools did not constitute equal educational opportunity and violated non-English speaking students' civil rights. The supreme court unanimously decided that regardless of one's ability to speak English, public schools must allow all students to participate. Although this case was a huge milestone for the progress of non-English speaking people in the United States the court limited it ruling by stating that schools did not necessarily have to provide supplemental language instruction. 
In 1981, the court revisited the issue of non-English speaking children in public schools. In Castaneda v Pickard, more specific guidelines were set to ensure that non-English speaking children were still receiving an equal education. This case set forth three guidelines that all schools must meet: "the bilingual education program must be 'based on sound educational theory,' must be 'implemented effectively with resources for personnel, instruction materials, and space,' and after a trial period, the program must be proven effective in overcoming language barriers/handicaps." 
Some states have mandated what methods of teaching are available to their students and for what lengths of time. In California, where nearly a quarter of students are ELLs, Proposition 227, passed in 1998, requires that, with a few exceptions "all children in California public schools shall be taught English by being taught in English. In particular, this shall require that all children be placed in English language classrooms." It does allow for a transition period, but that is not meant to exceed one year.  Massachusetts and Arizona have similar laws. 
Models for Teaching ELLs
Models for teaching English Language Learners can be broadly categorized into three groups: English-Only, True Bilingual, and Transitional Bilingual. In English-Only models the students' native language(s) are used sparingly, if at all, and the focus is on structuring the english content so it is more easily understood. In True Bilingual education, students are taught both in english and in their native language(s), and there is a focus on maintaining literally in their native language(s). Native English speakers may be included in bilingual programs as well, and these programs are found more frequently in Elementary education. Transitional Bilingual programs teach students content in their native language, as well as teaching English skills, with a focus on transitioning as soon as possible to education in English.