I don't think it's ever actually come up, but given the number of organizations - both governmental and NGO - that are specifically tasked with dealing with metahumans (either positively or negatively) there's likely at least one classification system - if not multiple ones (ie, NOWHERE uses a different system than the DEO, which uses a different one than STAR Labs)...which, actually given the differing focuses of those various organizations, would make sense - for instance NOWHERE would have a classification based on their usefulness, the DEO based on their potential danger to others, STAR on how it effects the meta themselves...

Interestingly, the books I'd have expected to see it recently - Captain Atom, Frankenstein, Superboy, and Teen Titans - don't seem to have done anything at all on that front.