When a girl dreams like this, it is possible that you feel some kind of threat and pressure from that classmate, such as emotion, study, life and so on. Anyway, you are a little unhappy with that classmate. Dreams are the fulfillment of wishes. In reality, wishes are often realized through hints and metaphors in dreams.
Dreaming that a classmate was drowned twice in a dream doesn't mean that you want her (him) to be drowned in reality. But one thing is certain, you have secretly or subconsciously rejected her (him).
Don't tell your dream to that classmate, just pretend you didn't do it. It doesn't matter.