This article makes no mention of what they're poisoned with, or how. It's entirely your assumption it's due to school supplied food.

Poisoning is VERY hard to trace, one of the most famous examples was the Chicago Tylenol Murders back in 1982. 7 people were killed, it costs J&J millions if not billions of dollars, multiple police agencies and the FBI were involved, and they still haven't identified a suspect.

In order for someone to be punished, they first have to be identified and apprehended. It's silly to assume a blind eye is being turned just because the investigation is still ongoing.