This is an art historians' label used to refer to a group of artists or paintings of the same geographical origin, and with stylistic similarities.
For example, painters described as of the Paduan School are those who worked in or near the Italian city of Padua. The term does not imply that these artists necessarily trained together (although they may have in some cases). In a broader sense it is used to group artists by nationality, hence phrases like 'the French School' and 'paintings of the Spanish School'.