Anything that's precatory has something to do with hoping or wishing for something, rather than requiring it. If your school's dress code is precatory, it's more of a suggestion than a hard and fast rule.
Precatory is a formal (and rare) adjective that means "entreating" or "requesting." A precatory list of school supplies, for example, would be things the teacher hopes each student will bring on the first day, rather than required items. You're more likely to find this word in legal documents, especially wills, where it distinguishes something legally binding from something simply expressing a hope: "The provision made for care of her seventeen cats by her niece is merely precatory."