The key word there is ‘currency’; which is to say the universal symbol of value that allows one to trade what they have to offer without demanding some kind of direct relationship between the parties. (My chicken for your shoes, for example, only when I need shoes and you need a chicken.) Students leaving school have lots to offer, but other than the diploma itself, grades are the only real symbol of the value of what they have. (Of course, that diploma also CAN have tremendous value, particularly if attached to a powerful institution — hence our current admissions scandal.)