WordNet sense 1 (the period of instruction in a school; the time period when schools is in session):

schooltime, school day

WordNet sense 2 (a large group of fish):

shoal

WordNet sense 3 (the process of being formally educated at a school):

schooling

WordNet sense 4 (a building where young people receive education):

schoolhouse

WordNet sense 5 (train to be discriminative in taste or judgment):

civilize, cultivate, educate, train, civilise