I’d bet the farm that the reason the schools went to foam was because some do good meddler calculated how much water was going down the drain with the washing of reusable trays.
Foam is for takeout meals. I am fairly sure it is more cost effective for any restaurant to wash dishes. Fast food places excluded.
So restaurants can get their act together but schools that serve a thousand or more students will not and use foam dinnerware
See the very nice lunches French school children get and served on plates of course
http://www.wimp.com/schoollunches/