Floral is okay. In my opinion a conservative cut/length is more important than the pattern, although you might try to avoid something too loud or garish. Girls wore all sorts of dresses to my school's ceremony, ranging from maybe-too-short to bright colors to "wear-to-work"ish outfits, though a year later I can't remember the specifics of what anyone in particular wore lol. Just pick something you feel comfortable in.
Unfortunately our school mandated that we all wear a "white top with black skirt or pants" for whatever reason so I'm jealous of your ability to choose! Had I been able to dress myself like the adult that I am, would've gone with a classic sheath dress - flattering and conservative. Florals or solid, both would work! Enjoy