The effects of laws on EL learners has been dramatic. The final determination on whether the dramatic effect has been positive or negative, however, is hotly debated. Many states have changed their laws to either compel schools to provide classes in both Spanish (usually it is Spanish speaking students involved) and English in the thought that at least while they are being taught English they can continue to learn other subjects in their native languages. Both NJ and NY have such laws. Other states, such as California and Texas now forbid classroom instruction in anything but English.
The main law that affects EL learners can be found in the US Constitution, the Due Process Clause of the Fourteenth Amendment. This Amendment says states cannot provide disparate (or non-equal) treatment to all state residents. Therefore, all must obtain a free education in exactly the same way, whether or not they speak English. As you can see, the 14th Amendment can be used to either deny the right to classes in other languages (if all get English only education then it is equal) or to support other language instruction (all cannot be said to be getting an education if they cannot understand the teachers). The individual state laws, those which enforce or prevent other language instruction have different effects.
In many respects the idea to teach in any language but English can be said to slow down or even completely prevent the process of learning English. The younger a child is when he or she is taught a new language the faster and more easily he or she will learn it. In this way, placing a kindergartener through 2nd grader in a non-English only system may prevent that child from ever fully learning English. For older children, those past age 8 or so the process of learning another langauge become harder and slower. It may be unfair to deny these children any education until the time they do learn some English, which, sadly, is what state laws forcing English only instruction may do. However, the problem of non-English only instruction for an older child may become the fact that learning a second language is difficult and if these older children do not see or feel the need to learn English, then non-English only instruction may allow them to avoid learning English.
Unfortunately, as the above describes, neither extreme is good on its own. What EL instruction should do, instead of forcing English only or other langauge instruction only, is use educational and psychological studies to determine the ages at which either approach is best and then base EL education on that principle. Also, to help motivate older students to learn English, put some kind of deadline on them so that they must show English language improvement in reading, writing, or communication throughout the year or they will be "left back."