Entering the Secret Realm
Xue Bai's breakthrough was swift, as he wasn't very far from the peak of the Bone Strengthening realm. But as he did break through, he did cause a small uproar.
It was very hard for people to notice if someone was cultivating the Purification Stages as if even if you could see through their cultivation, all you would see was a person at the peak of a realm.
This happened with Xue Bai. Once the students saw him, they just assumed that he was at the 9th stage of the Bone Strengthening realm, not the 12th. It was only once he broke through that they realized he wasn't.
It was a simple realization.
If a cultivator was at the peak of a realm and broke through a stage but somehow stayed in their realm, wasn't it obvious that he was cultivating the Purification Stages?