My sister once ----- as a school teacher.

The best verb tense for this sentence is: worked.

Here's why:

• Work is the present tense, indicating an ongoing action. However, your statement suggests your sister's time as a teacher is in the past.
• Works is also present tense, implying your sister currently teaches.
• Working is the present participle, which describes an ongoing action. Similar to "work," it wouldn't be suitable for indicating a past event.
• Worked is the past tense, which perfectly captures the idea that your sister used to be a teacher but may not be anymore.

Therefore, "worked" accurately reflects the past action of your sister being a school teacher.