If your student loan is at 7.9% or 6.8% then you probably ought to just pay it off directly before you think about buying bonds.
What you're proposing is equivalent to investing on margin, aka leverage. Some brokers offer margin accounts at interest rates of about 2% or so, using the assets you buy as collateral. Generally, leverage is a way to increase your returns by accepting higher risks, it may make sense for some people depending on their risk tolerance, but it seems odd to lever low-risk/low-return bonds rather than investing in stock or real estate.
Also, most bonds aren't "cash-equivalent". One-year US Treasury bonds are probably most acceptable - but the return on this kind of "cash-equivalent" investment is so low that, even if you are making more than the 2% you're borrowing at, it just doesn't make sense to pay only $900 on your high-interest loan, instead of the full $1000.
Think about it - the bank is offering you a rate as low as 2% because your collateral makes it super-safe - if you don't pay back on time, they'll definitely be able to get their money by seizing your collateral. In this sense, the bank's investment in its loan to you is itself cash-equivalent. Hence, a reasonable return to expect on your cash-equivalent investment would be pretty much the same interest rate that you're borrowing at, so it's probably a wash.