What kind of unclear reference does the following sentence contain? In this school district, they require uniforms for all grade levels above fifth grade.

There is no unclear reference in the sentence. The only pronoun in the sentence (they) clearly refers to 'this school district'. Perhaps the problem with the sentence is that a plural pronoun (they) is taking the place of a singular antecedent (this school district). However, it is often acceptable to use a plural pronoun to take the place of a singular noun that is a word for a number of people or things. Examples: The staff is off today. They are at a seminar. The team will be a little late. Their bus broke down. The herd got out of control. They trampled the corn field.