Your concerns are very valid. If possible meet with the teacher again to have him or her explain why this particular material, that you feel is too difficult, is taught at this level. If you are not satisfied with the teacher's explanation you have every right to go to the school principal.
If your child is in a public school, the first grade curriculum is aligned with the National Educational Standards, which are age and grade appropriate. If your child is in a non-public school its curriculum might be different.