The purpose of this study is to research the differences in cognition between English teachers and students on "separating classes by proficiency level" and "differentiated instruction in mixed-level classes". For this research, a survey was conducted on 117 English teachers and 153 students. The study shows some results in three major categories as follows; first, teachers and students" opinions on students" level differences; next, their cognition on separating classes by proficiency level; and finally, their cognition on differentiated instruction in mixed-level classes. Most teachers (more than 90%) and many students (almost 68%) cognize that there are English level differences among students. More than two thirds of teachers consider separating classes by proficiency level to be helpful for students, but just less than half of students (45%) think that they need this. For differentiated instruction in mixed-level classes, almost 90% teachers have positive thinking, while only 30% of students feel its necessity. Furthermore, in respect to the management of "differentiated instruction in mixed-level classes", teachers answered that the main problem is difficulties in management while many students pointed to the problem of grouping and conflicts among different level groups.