Answer:
The 3rd-grade teacher should say that the school systems and ways of teaching get updated each year.
Explanation:
Children's parent's schools would have probably taught different techniques and ways of learning a topic. As years go by, more efficient ways of learning that topic are presented and selected.
If the parent would like the child to learn that way, they may teach them at home, but the school has decided on a system for at-school learning that seems to have worked the best.
Hopefully, that should answer the parent's questions!