2、来自古英语 sceald,浅的，词源同 shallow.
4、来自古英语 scolu,鱼群，词源同 school,鱼群，一群。
2、shoal: English has two distinct words shoal. ‘Shallow area’  is descended from the Old English adjective sceald ‘shallow’, which in turn came from prehistoric Germanic *skaldaz. (English shallow  is related, although it is not clear precisely how.) Shoal of fish  is simply a reborrowing of Middle Dutch schōle, which had earlier been taken over as school.=> school
4、"place of shallow water," c. 1300, from Old English schealde (adj.), from sceald "shallow," from Proto-Germanic *skala- (cognates: Swedish skäll "thin;" Low German schol, Frisian skol "not deep"), of uncertain origin. The terminal -d was dropped 16c.
6、"large number" (especially of fish), 1570s, apparently identical with Old English scolu "band, troop, crowd of fish" (see school (n.2)); but perhaps rather a 16c. adoption of cognate Middle Dutch schole.
8、"assemble in a multitude," c. 1600, from shoal (n.2). Related: Shoaled; shoaling.