Princeton University’s endowment earned an investment return of 0.8 percent in a year when many schools posted losses as they grappled with a volatile market. The fund’s value decreased about 2 percent to $22.2 billion as of June 30, the New Jersey-based school said in a statement Friday. Princeton and Yale University were the only colleges among the eight Ivy League schools with a positive return. The group’s weighted average performance was a decline of 0.2 percent, according to data compiled by Bloomberg.