The only thing the school can do is report them to a school, and/or a DSS social worker if it's that serious, and possibly recommend a child nutritionist if the parent(s) don't already have a PCP for their child/children.
There's not much else that can be done, it's not a crime to feed your children even if it is unhealthy foods, it's better to feed them than not at all

, that's how it'd look from a DSS social worker's perspective, I know this because one of my aunties has been a social worker for nearly 15 years, and most of them would rather have cases like this than the other way around.
This doesn't mean they're in support of unhealthy eating in children but it could be much worse and has been in the past many times with cases such as starvation, malnutrition, etc.