Explanation:

- She used to: This phrase is grammatically correct. It indicates a habit or action in the past.

- Teach dance: This segment is also correct. "Teach" is a verb and "dance" acts as the object, forming a proper verb-object phrase.

- In a school: This is a correct prepositional phrase. It specifies where the teaching took place.

- Did she?: This is where the grammatical error lies. The sentence starts with "She used to," which means the correct question tag should be "didn't she?" Generally, positive statements take negative tags and vice versa.

Correct Answer: Option: 1, did she