Here's a revised version of your passage with improved clarity, grammar, and flow while keeping the tone and intent intact:
The classroom door creaked open. Instantly, all the students rushed to their desks. Standing in the doorway was our homeroom teacher—Mr. Ethan. His reputation at school was far from stellar, and somehow, he always seemed to treat me ten times worse than everyone else.
Then, unexpectedly, I heard a voice from the demon lord.
"Of all people… it had to be him," he muttered.
I turned to him, confused. "You know him?" I whispered.
He smirked. "You don't need to whisper—I can hear your thoughts."
That realization hit me like a brick. All this time, I could've just thought what I wanted to say. Worse still, anyone watching me whisper to myself probably thought I was insane.
He chuckled. "Don't worry. I can only hear your surface thoughts, not your memories."
Still curious, I asked, "How do you know Mr. Ethan?"
He gave me a devilish grin. "Truth is, I only heard rumors about him back in the day. Believe it or not, that man was once a well-known con artist."
I frowned in disbelief.
"Yes," he continued. "He pretended to be a hero using his family's wealth. He wasn't a real candidate—just a fraud. Most people don't know because his family paid handsomely to keep it quiet. Besides, it was a chaotic time, with new heroes popping up every month. Honestly, I think there are still some who never even made it into the history books."
Suddenly, it all clicked. Mr. Ethan never helped us with training—he only handled the general education classes. And I had heard whispers about his family making large donations to the school.
Mr. Ethan finally spoke, striding to his desk with a sour expression. "The school competition begins next week. I hope you're all prepared." His eyes locked onto me. "Especially you, Mr. Saber. Do you know what happens if your results aren't good enough?"
He flashed a malicious grin.
"You'll be expelled from Class A… and reassigned to Class D."