At Kuroda Elementary School, students gather at a set time and walk to school as a group.
Literal
Kuroda elementary-school [at-では] decided time [at-に] gather-[and-て], group-going-to-school do-[polite-します].
集団登校 ('group commuting to school') is a Japanese elementary-school practice where students walk to school in groups from their neighborhood, for safety and community-building. 決められた ('decided, set') is the passive-past attributive form of 決める. This kind of communal practice is a cultural feature of Japanese primary education.