The average return, after fees, on college and university endowments was 15.8 percent in the fiscal year that ended in June, up from 11.7 percent the previous year, according to the preliminary data collected for the annual Nacubo-Commonfund Study of Endowments, which will be released in January. The largest endowments — those over $1 billion — had the highest average return, 16.8 percent. The study, based on data from 426 colleges, found that colleges were allocating more than half their investments — and almost two-thirds of the largest endowments — to alternative strategies such as hedge funds and private equity.