As class starts winding down, children naturally return to their seats and quiet down, even though the teacher doesn't say a single 'be quiet' or 'sit down!'
Literal
Class [gen-の] end near become-[when-と], teacher [subject-が] one-word-also 'be-quiet' 'sit-down!' [quotative-と] not-saying [despite-にもかかわらず], naturally children [topic-は] self [gen-の] seat [to-に] return quiet-become.
An observational sentence about classroom dynamics. ~と is the natural-consequence conditional ('when X, Y happens'). 一言も~言わない is the 'not a single word' pattern: counter + も in negative. ~にもかかわらず ('despite X, Y') is a formal concessive conjunction, more weighty than のに or けれど. 自然と ('naturally, spontaneously') is an adverb of spontaneous action. The whole structure models the formal-observational tone of educational writing.