Searching for clue and evidence
Firstly, before thinking about doing anything else or even think of reporting they decided to first of all look for places Bernard and Sophia had been before in order to get clues and evidence, they didn't want to carelessly accuse the second years after all if they just decided to go report like that, the second years could simply just deny and say the trio were accusing them without evidence.
Which could prove to be bad considering how strong the second years were and how deep their influence was in the academy they could just attack them afterward, and no one would be able to say anything rather there would even be students and teachers who would support and vouch for them.