ask them to speak only in English in the class room

Bui, that is not my field, per se, but when we train deaf students to go from manual to oral communication when they receive cochlear implants, we measure their progress by increased working vocabulary and spontaneity. I would think that we have in the kind of setting to which you refer, vocabulary, syntax, and sentence construction that are measured at various points of development. Age appropriate vocabulary would be in that mix, I would think.

Speaking English only in class is definitely a first consideration. However, I'm teaching 2 different 6th grade classes and each experience is drastically different. In class A, the students have VERY limited L2 exposure by only having 2 hours of their schedule in English.  They have a shortage of decent resources and their vocabulary is limited to the point where I am forced to speak English and Portuguese or they don't understand anything.  It is too early to simply shut out their L1 at once and go straight L2, but my goal in 2 months is to curb the Portuguese altogether. Class B is from a school in a well off neighborhood here in São Paulo, Brasil.  They are also a heterogeneous group but their ability to learn is much more advanced.  I don't speak Portuguese with them at all and their overall ability as a class is far superior to class A.  It could be a demographic issue or it could be my fault with group A... But I am trying to advance them(class A) along by having HW everyday for 30-60 minutes of reading, writing, speaking and listening. I am trying to get them to focus on a skill each month and raise their awareness and vocabulary.  I started with a placement test to see where they are and if you email me, I will gladly send you my tests. They are not overly difficult but the class A had some difficulties with it. I will also assess with an oral exam in December before they move up a level...I want to assure that they have mastered the info at that level verbally.

dear James Winter , i appreciate your effort

it is demography , parents , and overall environment matters

I like role plays. They are closely connected to the real life situations and you can see if your pupils can speak appropriately to the chosen topic.

Dear all, thank you for the replies. I already had some placement tests for my students. However I wonder how to evaluate their level. In another words, which criteria to evaluate their speaking competence? Could you please share with me what is speaking competence?

To "James Winter": It's great to see your oral test for your students. Please send to me via this email: [email protected] . Did you base on any official documents to design those tests? Thank you very much.

Bui Minh, is your main goal to get your students to pass a test or to communicate effectively?

If your goal is effective communication - and you have the time - ask them to have a brief conversation (2-5 minutes) in pairs or triads in English. Give a vague prompt such as "talk about your families."

If your goal is to have them pass a test, give them a task which is similar to a major part of that test.

Competence is a big word. This concept needs to be defined before you start. Some can articulate their ideas well, others can speak in a conversation very well while not putting more complex ideas or changes in topics well. 

As was suggested gve som starters and have students have a conversation. See if they can transition to topics, have extended sentences, and as quality questions. If they can paulthis without extra prompting then add the idea of grammar structure.