The crossword question hints at an object that was widely used in the past and had the characteristics similar to that of a tablet. The answer to that question is “SLATE“. In the olden times, slates were widely used in schools as a writing surface. Students would write on these slates using chalks and then wipe them clean with a cloth. The slates provided convenience in terms of reusability and affordability. Therefore, the term “old school tablet” is used as a metaphor for “slate“.