They had been dancing / for an hour / when our dance teacher / had arrived.

The sentence uses the past perfect continuous tense ("had been dancing") to describe an action that was ongoing in the past. To indicate a simple past action that occurred afterward, the correct tense is the simple past ("arrived") rather than the past perfect ("had arrived"). The past perfect tense is unnecessary here because the action of the teacher arriving is not an action completed before another past action but rather a concurrent or subsequent event.