In Bel Kaufman's Up the Down Staircase, part of the story is told through notes that Sylvia Barrett sends to other teachers. For example, the first one goes
FROM: Mrs. Beatrice Schachter, Room 508
TO: Miss Sylvia Barrett, Room 304
Welcome to the fold! I hope it goes well with you on this, your first day. If you need help, just holler; I'm in 508.
What's your program? Can we synchronize our lunch periods?
In chapter 2, Sylvia and Beatrice seem to exchange seven notes within a single day of work.
How were they sent? Did they ask students to deliver them during classes? During breaks? My imagination runs rampant and thinks of pneumatic tube transport, but surely this wasn't the case in 1960s US schools?