Most ESL students do well in science and many say that it is their favourite subject. There are good reasons for this. Firstly, science is based on certain universal principles and procedures. So, very often students will be familiar with science lab practices; they may also have some prior knowledge in the particular field being studied. Secondly, science is a subject that generally advances from the concrete to the abstract. Students are first required to observe and measure before being asked to hypothesize and analyze. And thirdly, much work in the science lab is done in small groups. This maximizes the ESL students chance to engage in meaningful interactions with their peers and the teacher, thus improving their chances both of learning English and of learning science.
So, it seems that the very nature of the science class is ideal for the ESL student! The advice to the teacher can be summarized as follows:
There is an excellent extended account of how the science teacher can help the ESL students in her class at: http://www.ncbe.gwu.edu/ncbepubs/pigs/pig11.htm