Go to the office of admissions and let them know you want to sign your child up for the class...More than likely they will just throw your child into the right class....

All international schools will have different admissions policies this may include things such as a wait list or standby.

All of the major cities in China have numerous "international schools", some of higher standing than others.  Bear in mind that such international schools are not cheap by any means, particularly in Shanghai and in Beijing, and particularly if you are looking at one of the quality institutions, as such may exist.

Remember, however, that most international schools are private institutions, and thus are driven by profit rather than by education.  If you are in Shanghai or in Beijing, you may wish to consider some of the International Sections of the better government schools.  They do exist and their tuition rates are less than the "international schools" you may be seeking.  They usually offer stricter academic guidelines as they are more closely regulated.

But to answer your question, as someone said, simply call the school, make an appointment and go see for yourself.  Keep your eyes wide open, however, and in the bigger cities, I would advise shopping around until you find the right school.

Do not underestimate visiting the school( in your city. Make sure to mention if your company is paying their tuition or if you are solely responsible for coming up with the funds.