The earliest form of a notebook was the wax tablet.
Wax tablets provided a portable, reusable writing surface in classical antiquity and throughout the Middle Ages. A writer could scratch marks into the wax and later smooth the surface to use it again, making the object practical for notes and temporary writing.
Paper gradually became more available in Europe from the 11th century onward, and wax tablets slowly declined. They remained relatively common in England for longer because England did not have a commercially successful paper mill until the late 16th century.
A clay tablet is an older writing technology, but it is not the earliest form identified in the history of the notebook. Modern school notebooks developed much later from bound or folded paper. During the Enlightenment, British schoolchildren were even taught to make notebooks from loose sheets by folding, piercing, gathering, sewing, or binding them.