。
She's often late for school on Mondays.
Literal
She [topic-は] often Monday [on-に] school [at-に] is-late.
しばしば ('often, frequently') is a slightly written-feeling adverb; in casual speech you'd more often hear よく ('often'). The verb 遅刻する takes に for the event or destination one is late for — 学校に遅刻する ('late for school'), 会議に遅刻する ('late for the meeting'). The day-of-the-week (月曜日) also takes に for the time-when. Cultural angle: punctuality is taken seriously in Japanese schools, with chronic tardiness flagged as a 生活指導 (student-life guidance) concern; teachers track it carefully, and parents may be called in for repeated late arrivals.