There are a handful of tell-tale signs a school is facing stress. Rider University, a landmark in central New Jersey for the past 158 years, has been grappling with most of them. The school is losing money. Over the past decade, it has lost about a fifth of its student body. And this month, the outlook for its credit rating, already below investment grade, was cut to negative. Gregory Dell’Omo, Rider’s president, has been working hard to stabilize the university’s finances. He’s laid off staff, cut programs, even put a crown jewel, the school’s Princeton campus, on the auction block.