Percy could have quite literally opened the flood gates, but that would have meant that he and Groot would have had to make a run for it before the water became noticeable. Annabeth always said that he didn't think things through enough, she might have been proud at his forethought. Even if it was to essentially cause destruction. Percy would have taken a detention happily, but he also didn't want to out himself so soon. Not to the teachers, anyway.
"Yeah, I guess I am. Met some cool people here, though," he nodded and pulled down a vine from where it was before tossing it into the bin.