Your state's Department of Education may have the math standards per grade on its website. I would also suggest talking to the teacher about the rumor you heard, and learning more about the math curriculum being taught in the classroom directly from the teacher. I'm including some links below that you may also find helpful.
I would not be surprised, I also know someone whose child was in a private school and later when assessed to enter public school, did not perform to grade level and actually had to repeat the grade.
What I think is important to ask yourself - 1. Is your child learning? 2. Is your child happy in their environment with the current school?
If your child is learning and is happy, I would not concern myself with rather or not they are performing on the grade level compared to public school. Much of the material that is covered in public school is slightly not age appropriate. This can be covered in later grades at which time it would be more beneficial. Your child may not have covered the same material as set by the state, but may be spending more time mastering their basic skills, which may be a more powerful aid later.
Meanwhile, if you are interested in introducing your child to other material, you can work on it at home. Use these great sites to acquire material to use at home. You may also, want to do a internet search on the local schools and go to that school's webpage to see what they are teaching on grade level. My school has a webpage that I can go on and actually go into the different grades by teacher to see what they are currently studying. This may aid you in what to teach your child or what the others in public school are learning.