It's 2 parts:
1) There are more programs for those that are in school (daycare subsidies, etc.)
2) In the income/outgo part school money counts differently. Take my H and I. When my son was a baby we were both in school. Our "income" was between 40-50k. BUT 9/10ths of that was student loans and scholarships. As a family of 3 we would have been x'd out of WIC, BUT because they were loans and scholarships ALL of that money was taken out of the equation. Which made us not only qualified to be on WIC, but highly qualified. If a person earns 50k and decides to spend 25k of it on school, their income isn't adjusted to 25k. But if instead that 50k is all scholarship money (like ours was) we were REQUIRED to be in school in order to receive it, their income IS adjusted. Each of us got between 15-20k in scholarships. We had about 12k a year from work. WIC counted the 12k and not the scholarships and loans.