There is an inverse relationship between bond ratings and the required return on a bond. The required return is lowest for AAA rated bonds, and required returns increase as the ratings get lower. (worse)
"Treasury notes are negotiated at higher prices than those of corporate bonds because there is a greater risk of default with bonds. That is why prices are estimated charging them with the highest discount rate (or yield-to-maturity). Then you require of a greater coupon yield (and ...
This posting answers the student's question on bond ratings and gives references for the same.