、。
Right at that moment, the two in question showed up at school.
Literal
Exactly that time, aforementioned [gen-の] two-people [subj-が] came-to-school-[and-てきた].
件の (くだんの) 'the aforementioned / the X in question' is a formal/literary demonstrative, sounding slightly stiff in conversation but very common in narrative and business prose. ちょうどその時 'right at that time' is the adverbial phrase for narrative timing. 登校する 'go to school' is a compound verb. ~てくる here is the 'come / showed up' sense — motion toward the observer's viewpoint.