。
She enrolled in this school last year.
Literal
She [topic-は] last-year this school [into-に] enrolled.
入学する ('to enroll, enter a school') is the formal Sino-Japanese verb for matriculating — distinct from 入る ('enter') and 通う ('attend'). It marks the institutional act of joining the school as a member. Because Japan's academic year begins in April, '昨年入学した' typically points to last spring; this one phrase carries an implicit calendar that English speakers don't share, where school years start in autumn. The に here marks the school as the destination of entry.