Alicia would be shyly really happy and touched, especially if he took the time to get the specific type of flowers she likes, because she's not a fan of roses and her tastes are more niche. (marigolds, geraniums, etc.)
Kattar, despite being a boy, loves flowers and is not ashamed of that. His favorites are peonies, because pink is his favorite color and they look so delicate and sweet. (their scent bothers Alicia though. She's not a fan of the smell of flowers and Peonies have a very strong perfume.) If Alicia bought him peonies he'd be super touched.
Oh wait...:

Art by Mariel Leister
Jinwoo: He loves flowers, so if someone he liked bought him flowers he'd be super happy (though he might forget to say thank you, depending on how surprised he was at receiving them.) He also would probably not recognize it as a confession of love.
Kimberly: Would be really happy, surprised and flushed at the same time, but also a little uncomfortable.