All classes are 'mixed ability' classes.
All classes consist of individual students with different personalities and interests.
All students also, themselves, have 'mixed abilities'. For example, some students may find writing easier than speaking, or vice versa. Some students find one particular task or approach more appealing than other tasks or approaches. The second aspect refers to their ability to learn. A student may be weak in English, for example, but given appropriate support may be able to learn quickly.