What did naughty boys in past generations dip pigtails into?

Inkwells

This was when schoolchildren learned to write with ink pens that had to be dipped into inkwells to replenish. That was before my time, but the desks in the grade school that I attended all had circular holes near the upper right corners, to accommodate the inkwells that were needed. You could see the stains near and within these holes, the result of years and years of diligent learning.