I work at an elementary school and part of my job is the lunch money, etc. So, for my school, I would do two things: one, talk to the teacher and ask if your son can bring his milk carton home (maybe provide a plastic bag), then you can see the expiration dates for yourself. And two, I would send contact the principal and the district food services director.
The principal has no say in what is sent to your school, but the principal can have the school staff look more closely at the dates and then they will know not to serve expired milk. Also, if they know a parent is concerned, and if they are doing their jobs, they will work to make sure it doesn't happen again.
The district food services director should then be able to contact their milk supplier and be able to make sure they aren't delivered expired milk.
Another thing to consider, some companies are switching their dating system to the day/month/year, instead of what we are used to seeing which is month/day/ year. So the date could have been the 2nd day of _____. Does that make sense?
I'm sorry this happened to you. Good luck!