From what I understand of the situation, the students had bought the fish the night before, and brought them to school in plastic bags which they kept in there pockets. During 4th hour they were attending swimming for gym and released the fish while they were swimming in the pool.
"Don't want to sound like a fanboy, but I am with you. I'll buy it for sure, it's just a matter of for how long I will be playing it..."
- Silvast, Battle.net forums
I dislike swimming with children.