Growing up, there wasn't usually much reason for anyone who wasn't taking notes to pay much attention to the chalkboard.
If the teacher wasn't writing something on it that was directly related to the lesson, it was probably the name of some kid who behaved badly enough to stay inside at recess. Bonus points if that kid earned this punishment for messing around with the chalk erasers.
But of course, that's likely been true for as long as schools have been set up the way they are. So, we can only imagine how interested someone 100 years into the future would be if they made a similar discovery to construction workers at one Oklahoma school.