Snehk's Dreamlands

I was in school. The classroom was heavily crowded. One of my classmates was having an exam at English lesson. He considered one question, but then another classmate whispered something, waved his hand and a puff of gray smoke shouted out of his hand, and striked the exam classmate on the head. This removed all of his knowledge from his head. Rest of us laughed. I recall that during casting, he muttered "Domus durbentia -'Dementia!"