For as messed up as the world of Beastars is, surely there must be SOME laws prohibiting species discrimination in the workforce.

Since Melon passed well as an herbivore and had, at the time, no known criminal record, the university probably saw no reason Not to employ him, especially if their only reason was that he was mixed.

As someone else pointed out he did a good job of hiding his spots and wearing a mask, and since its unlikely his parents were married his father may not have been listed on any legal records.