In A Separate Peace why does Mr. Ludsbury stop Gene, and who had called him?
In chapter six, Gene has a fight with Quackenbush--they exchange blows, and end up falling in the river together. Gene was soaking wet from the incident, and as he tried to get back to his room unnoticed, Mr. Ludsbury intercepted him and started questioning him about where he had been. Gene said that he accidentally fell in the river. Ludsbury took the opportunity to then drill him about some things that he was dissatisfied with: rumored "gaming" that had been going on in the dorms during the summer, Gene's slacking in his studies, and how disrepectful they had all been about following the rules and helping out around campus. Gene stands there and guiltily takes the lecture, because he feels awful for what happened to Finny, and realized that he had ruined the entire summer with his bitterness.
The real reason that Ludsbury stopped Gene though was because someone had called for him. The lecture was just a side-note that Gene's doused condition had sparked. After the tangent of a lecture, Ludsbury tells him he has a call, and lets Gene go return it. It is Finny--he called to tell Gene to hold his spot as a roommate, and that he was going to help Finny play sports by playing them for him.
I hope that those thoughts helped; good luck!