No one but (she/her) ever made a perfect score on the test. Please explain the general rule regarding the proper use of pronouns in such cases.

When a noun or pronoun is used as the subject of a verb, it is said to be in the Nominative case. When a noun or pronoun is used as the object of a verb, it is said to be in the Objective or accusative case. Also, a noun which comes after a preposition is said to be in the accusative case. "she" is used when pronoun is the subject of a sentence and her is used when the pronoun is the object of the sentence. The correct answer will be "her" because it is coming after a preposition "but".

No one but "her" ever made a perfect score on the test is the correct usage. Shrishti's explanation is correct.