Aside from taxes, there are lots of other expenses involved with attending public school. Here's my short list: back to school clothes, school supplies, lunch money, pictures, field trips, donations to PTA and charities, gifts for teachers, bus drivers, and crossing guards. I didn't discover how expensive "free" public education was until my son entered Kindergarten.
Now, I am much more prepared. I set aside money each month in what I call a "School" account. This money is used for both kids for all those little expenses that come up over the course of the school year. Putting aside as little as $10 or $20 each month can help you avoid those "$100 mornings", so start now for next year.