Yes, this is correct, except that as a speaker of American I would not say '9th class'. I would say "Once you are in the 9th grade you will find out."
People from other countries would express the idea differently. I don't know whether any country would say 9th class, but perhaps that is the way they say it in your country.
Please see the thread PaulQ links to in post #9 for more discussion of this part of your question.