I did that too. I usually got milk in the Cafeteria for a nickle!
I did similar, My mother would pack my lunch, and learned, the hard way (me lambasting her at the end of the day) that you cannot put cocacola in a thermos. My lunch boxed were never the TV series ones, to my shagrin. I had one that was plaid and another that was in the shape of a mailbox that had the names of odd places and their zip codes, like Sunflower, Kansas, which no longer seems to have a P.O.
I would pay about 25 cents for a carton of milk, and every day, Lawrence Norman would ask me and everyone if he could have their "murlk".