What are the monthly salaries of teaching assistants and teachers working in a studio in Changsha?

It has little to do with the size of the studio. Mainly depends on your ability.

In a big studio with strict management, you have to pass the exam to become a teaching assistant.

Another point is not to mention your ability, it depends on what school you graduated from. With the same ability, the gap between graduating from the Academy of Fine Arts and graduating from ordinary schools is also very large.

Those whose monthly salary is above 15000 (my teacher used to be) are all good graduates with strong ability.

Only 1500.

It's usually around 3000. See ability.