First of all, your class size is more than optimal for language learning. Talk to your school administrators there to see if it is possible to reduce your class size.
While I was teaching in China, I taught a great deal of classes with between forty to sixty students. One class that stands out in my mind was with about thirty students: one student could not produce even one sentence in English while on the opposite end of the spectrum, another student was completely bilingual, accustomed to learning in the target language. Is your school grouping students according to their age or to their ability levels?
I recommend that you break your students into groups according to their language proficiency in the target language. This way, their existing knowledge of their native language should not play such a great deal. Arrange your physical classroom to reflect this methodology.
Have your students manufacture, produce, and create in the target language, giving open-ended instructions in the target language and assess their skills accordingly. This will initially create more pressure on you while planning, but overall, will lighten your course load. I advise you to create your classes in such a way so as to be able to shuffle mini-lessons among ability groups.
Several years ago, I was teaching Spanish in Mexico and in the school I found myself in, there were many ability levels in each class. By using this methodology, when a student is ready to progress to the next level, he or she is not hindered by his or her peers.
A clever metaphor might be a snowflake. Individual snowflake's compositions are unique, not unlike your students, and each flake could represent an ability level group in addition to your curriculum's structure. Do your students know what snow is or would they enjoy engaging in winter sports?
Since your students apparently already respect you, you shouldn't have trouble implementing new methods or strategies into your classroom.
Utilize the students with higher abilities to help you assess or train the students with lower proficiency levels. Seems that you are already doing this though... just trick them into learning.
Eric Paul Monroe