The loan is locked in once you take it. It never changes for the life of the loan, even if future loans have higher rates.
Further borrowing isn't the answer to education. Give every citizen a $25,000 interest bearing treasury account at birth for education after high school. Adjust this number upward every year based on the CPI less food and energy. By the time a person is 18, they'd have about $40,000, plenty good start on a college education.
Total cost last year would have been about $100 billion. Total Defense felt budget in 2014: $500 billion.