Navient Corp. has received a buyout offer worth about $3.2 billion from a pair of investors that the student-loan servicer rejected as too low.

Navient’s board said Monday that it voted to turn down the $12.50-a-share proposal from hedge fund Canyon Capital Advisors LLC and private-equity firm Platinum Equity Advisors LLC, believing it undervalues the company and is lacking in other ways.

The...