I don't know about the teachers here, but the school my boys attend has a very strict English only policy.  The students native language is only used if communication is very important and not understood in English.  (Most of the students in their school are either Japanese or Korean.)

I teach in a bilingual school and no, there is no English-only requirement in the classroom.  Such a requirement strikes me at time as totally counterproductive and somewhat ethnocentric. When I am teaching the little ones something more arcane, I don't want to waste 30 minutes giving them a little talk in English to make sure that they understand it when a Chinese explanation will suffice in 2 minutes.  It's just modesty of time over futility of effort.  The children do remarkably well.

I've been a little split on this one. The other foreign English teacher enforces this in his classroom, as in, as soon as they enter his class, they have to start speaking English, regardless of what they are saying, but I'm still not swayed. On the one hand, it is probably a good idea to get the students into an environment where they have to start speaking, and hopefully thinking, in the language exclusively. Using English only does have a tendency to cut down on the chatter that takes place on a natural level between students, also. On the other hand, some of my students proficientcy level is not that high, so I allow for a certain amount of native language use, in general. 

Personally, I don't enforce it if the conversation is before class, but once class starts I do try to make sure that the majority of the talking is in English. I don't allow students to translate for each other. If I ask a student a questin and they don't understand, we then back up and go over the question piecemeal, breaking down the words, grammer and context. This allows other students to benefit from the explaination, also. Then, of course, for assignments and oral excersises during the class.

So, I guess it is about 85% English (essentials), 15% Chinese in class (conversation among classmates).

Yes, violently!
Speaking is the most important skill in learning a language - to effect communication, and to show you've been misunderstood (and, thus, you need to fix what you say for it to be correct).

Also, it's those times they just want to 'chat' that is when they should be using their English the most - so that a) it becomes more natural and comfortable in daily life about daily things, b) so that they build up a vocabulary, and c) because it's the most relaxed situation to be speaking - so should be the easiest. To me, it's chatting with friends in the language that really marks one's ability to use it - not the stuff in exams!

(NB: on very rare occasions I allow Chinese for translation - usually the single word or concept. Also note, I 'teach' university students, so they should already know most of what we're doing, and are really only consolidating their skills)

For kids there is absolutely no point in rigidly insisting on L2. We go over class 'rules' with the kids every class, the first of which is 'No Chinese!'. However, there is, of course, a huge amount of L1 spoken, especially in craft activities. I'm just happy if I can teach new lexis, a new language structure and see both used in the controlled practice parts of my lessons (freer practice just isn't feasible with them yet).

If it's a well-behaved class, I allow Chinese for the first ten minutes or so, mostly so I can settle down the class and go through the new vocabulary (should I spend half an hour trying to do an interprative dance to communicate the idea of 'pragmatic' or just save time and say the Chinese word?). After I've gotten new vocabulary out of the way, I switch to 'English only' which works fine when I'm teaching kids, but is uninforceable with adults.

If it's a badly-behaved class, no way. If I don't use Chinese it's chaos and they learn nothing. I've seen schools where they stick to strickly to the 'English only' thing, and some of those classes are a complete waste of time and money.

I don't speak any Chinese at all in class. If the students want to use Chinese I give them some rope but am not afraid to reel it in. This hasn't presented itself as a problem so I see no reason to initiate such a policy.

Why would you have them stop what they are so natural at? They took those classes to learn something new not to get rid of what they already know. It's like me going to a math class and saying I can't use spanish in the class since sometimes they consider it two different languages. No!

I have taught a mixture of ages and abilities and have always worked by this general rule: if they have enough English to express what they want to say, then they must use English. So with an older, higher level class, it's mostly English, but with a younger class there's a lot more L1.

I have an English ONLY policy in my classroom.  That said, I frequently break it since "English" is composed of words from so many other languages.  I do not allow PU TONG HUA to be used unless I personally ok it. My students are in class to learn English not Pu Tong Hua.  If I hold up a image of an apple and someone says, "pingua" they find out very quickly how serious I am about it.  We have English only classrooms in The United States to teach US English.  Chinese teachers teach PU TONG HUA  without using English.  Yet inspite of these facts (and the fact that the methods most Chinese use to teach what they shamefully call English fail the students) this is a topic of such strong debate.  The method is called Language Immersion.

Actually as defined above, it would not be called Language Immersion but it would be called the Direct Method.  Language Immersion or Dual Language Immersion presupposes a slightly different approach.  If the poster above is working with flashcards first, and then the printed word second, theoretically that would probably fall under the Direct Method.