The One Where Ross Dates a Student - S6-E18
Other mistake: Ross' semester supposedly just ended, and he's handing in his grades. But the episode just before this one was about Valentine's Day, and the one immediately after this one is about spring break, so this episode is happening sometime between mid - February and March/April. It doesn't matter when the show was aired or produced; Valentine's day is still in February. There's no semester that ends at that time of the year, and NYU wasn't on the quarter system then.