Pennsylvania State University has undertaken a massive new recycling program — not for paper or plastics or food waste, but for the mammoth piles of stuff that students leave behind at the end of every school year, from sneakers to TVs to sofas. The end-of-term junk problem grew into a major headache on U.S. campuses during the 1990s, as students brought more and more stuff with them to school. Many colleges and universities just toss the left-behind items into dumpsters, but Albert Matyasovsky of Penn State couldn’t bear to see all those useful goods sent to landfills, so last year he funneled the detritus to the campus stadium for a massive flea market. This “trash-to-treasure” sale raised $15,000 for United Way and is now set to become an annual event. Other colleges are partnering with a Massachusetts nonprofit called Dump and Run that helps them set up a system for collecting student castoffs, selling them, and channeling the proceeds toward nonprofits.