Most, it not all, Chinese classes are mixed-ability. Some students may have done well on the Gaokao but this is not a real measure of their motivation or ability in an English language environment. Some will be bored, others highly motivated. Some will see the classes in English as relevant, others will respond in different ways. Where there is no streaming, banding or setting based on ability and testing, most teachers tend to teach to the middle of the range unless it is a one to one situation. In practice this means that the gifted students might get bored while those who are struggling might become disruptive or just switch-off.

Very good stuff. I thank you for posting this. Maybe you will find my experience teaching at Chinese state-run senior high schools of interest:

At those schools some classes consist mainly of motivated students. That is not an accident. It was engineered by the school in secret. The opinions of the pupils in those classes are taken seriously by the school hierarchy. If those pupils like you, you're accepted and respected by the school. If bad reports emanate from those talented classes, your job security is at risk. Because of this secret selection, there will be some classes with a lot of unmotivated pupils who will be difficult to teach. Essentially if you are unable to impart knowledge to them, it won't be held against you.

If you're teaching 20 classes of senior grade 1 students oral English, or senior grade 2 students oral English, but not grade 3 because they are in their gaokou year, you can expect 4 clever classes, 12 average classes and 4 unmotivated classes to teach. Such is the mix, and the 3 distinct groups will require varying approaches in your teaching methodology and content. The bottom classes will be harder to shut up before class begins and their classrooms will be untidy and littered with food wrappings! They will give you discipline problems!