。
She tends to be late to school.
Literal
She [topic-は] school [target-に] late tendency-is.
~がち ('tend to / be prone to') attaches to verb-stems and noun forms to express a general tendency, often a slightly negative one — 忘れがち ('forgetful'), 病気がち ('sickly'), 留守がち ('often away from home'). Here 遅れがち uses the verb stem of 遅れる ('to be late, lag') plus がち, giving 'tends to be late.' The construction is descriptive of habitual behavior, not a one-time event. 遅れがち differs in meaning from 遅刻する: 遅れる is the broader 'be late / lag behind,' while 遅刻 specifically marks tardiness for a commitment. With がち you'd typically reach for the broader verb form.