Rates on federal student loans have fallen to around 3 percent — a 35-year low. Even better, students can lock in those rates, potentially saving thousands of dollars by ensuring their payments won't increase even if interest rates do.But a proposal in Congress could shut down the party. The measure would end the fixed-rate option, making all federal student loans issued after July 2006 subject to variable rates. Repayments would then rise and fall each year in sync with interest rates.