。
I cross-dressed as a woman for the culture festival.
Literal
I [topic-は] culture-festival [at-で] woman-dressed.
文化祭 ('culture festival') is a classic Japanese school event — a school-wide festival where students put on performances, haunted houses, maid cafes, and cross-dressing photo booths. 女装する ('to cross-dress as a woman, dress in women's clothing') is a compound of 女 + 装. Cross-dressing is a common humorous feature of high school culture festivals, particularly in girls-dressing-boys or boys-dressing-girls photo ops.