I refer you all to the following article: Reexamining English Only in the ESL Classroom
Please read it carefully.
Working with teens that are forced to sit through English Only classes in the public schools, where all explanations are given in English, where these students are faced not only with understanding grammatical and linguistic concepts different from L1 but in a language that they do not understand, I find the "English Only" concept to be an inefficient use of class time. Considering that there is a very high failure rate among these students, considering that even after more than 12 years of English study these same students are unable to speak English, one has to question the efficiency of such an approach.
Most of my students, fortunately in small groups of 5 to 7 students, do not have enough of a level of English to gleefully sit through an hour of their ESL teacher saying "bla bla bla" in a language that they do not understand. They desire clear, concise explanations and rapid translations, they do not want mime work or twenty-five minutes dedicated in the communication of a concept that could be easily understood in twelve seconds by explaining it in L1. The majority of their time is spent on discovering different ways of expressing what they want to say and then beating those new ways into an effective way of saying the same in L2. With an average of only 55 hours of ESL class in an entire year, using L1 is an effective tool for class economy. The work of getting the language into their mouths is their own homework, they must do the practice in order to wrap their lips around those strings of sounds that make up the utterances that might later become useful to them in expressing their thoughts, ideas, desires.
Refusing those students the use of their own thought processes that are later reflected in their use of L1 removes from them the power of communication. They feel powerless in the face of not being able to express themselves in English, complicated by a prohibition to use their own language. Especially adult learners feel they are being lowered to a childish level, where their complex thoughts and opinions are discarded because they can not effectively express themselves in English.
Granted, in certain activities (language games to reinforce particular patterns or structures; role-playing to encourage universe of discourse understanding) an English Only rule might be effective. However, as can probably be noted from my comments here, I personally believe that restricting the language in the ESL classroom to English Only is not only not effective, it is also not at all supported by linguistic research into the question. Do a google search and read up on the subject before implementing the idea. Make your own decision, but do keep in mind class economy and the amount of work your students are going to do outside of the classroom. Too many jump onto the English Only bandwagon without looking into the social / political sources of this concept and the psychological impact it may have on the language learner. In the end, it is the language learner who must be seen to, not an unsupported claim that English Only is the Only English.
Thanks for reading.