Gov. Bruce Rauner’s administration says a pending move by Illinois’ largest public-pension fund would increase the state’s required payment by hundreds of millions of dollars, potentially leading to higher taxes or massive cuts to education and social services already suffering amid a budget crisis. The board of the Teachers Retirement System, which serves more than 400,000 teachers outside of Chicago, is scheduled to vote Friday on whether to lower its expected rate of return on investments. A reduction would trigger a larger contribution from the state, where the Republican governor and Democrat-led Legislature have gone more than a year without...