Using Transfiguration to turn the desk into Tom for teaching was, in fact, entirely a move of desperation for Sherlock.
He had only been familiarizing himself with the Original Owner's magical skills for about two months, and he had not yet been able to raise his level to that of an adult wizard. Based on the learning progress at Hogwarts, Sherlock estimated that his current level of magical understanding was roughly equivalent to that of a sixth-year student, though the number of spells he knew was significantly higher.
Therefore, while he understood many more flexible ways to use Transfiguration, he couldn't make the transformed objects overly detailed or lifelike. To avoid a poor imitation that would look ridiculous, Sherlock simply chose the cartoon character Tom from the start, whose appearance lacked fine detail and was very simple to transform.
In any case, as long as Tom possessed the magical effects he had added during the transformation, it was enough. Using the form of Tom could also slightly change the students' perception of him. At the very least, someone who liked watching Tom and Jerry certainly wouldn't be a strict and uptight professor like Snape.
As for finding a few real Dark creatures to use as teaching props, Sherlock had considered that as well. But as someone living cautiously within Hogwarts, he had no idea what channels to use to obtain such things. However, this matter might be resolved through Hagrid in the future.
The challenge in the classroom continued.
The successive failures of the Hufflepuff boy and Ron piqued the remaining young wizards' desire to win. When watching from the sidelines, they didn't feel Tom was particularly powerful; he was just a bit fast, had a flexible body, and possessed the ability to change shape at will, but nothing else seemed special.
The Tom that Sherlock had transformed was indeed not a very powerful creation. Not to mention the basic spell Incendio, learned in the fourth year, which could defeat it—even the general counter-spell Finite Incantatem, learned in the third year, could end the transformation, turning Tom back into an ordinary desk.
But unfortunately, this class consisted of second-year young wizards; they had mastered neither Incendio nor Finite Incantatem. Thus, only able to use monotonous spells against the enemy, they rotated challengers throughout the entire class, yet no one managed to land a single hit on Tom with their magic.
Near the end of the class, Sherlock turned Tom back into a desk, told the students to return to their seats, and summarized the issues for them.
"Tom isn't strong, as every one of you can feel, but why hasn't anyone been able to defeat it?"
"Is it because the spells you've learned aren't powerful enough? Or did the venue limit your performance?"
"Perhaps all of these are reasons, but in this classroom, is what you learned in your first year really insufficient to defeat it?"
"Or rather, do you think the magical knowledge you learned in your first year consists only of foolishly waving your wand and aiming spells at the enemy?"
"How to use what you've learned to defeat Tom—think about this for your homework. Go back and think carefully about how to put your ideas onto parchment. Your homework has no length limit; as long as it's effective, even a single word will do."
"Next class, you can challenge Tom again using the method you wrote in your homework. Those who succeed can still earn twenty points, but the exemption from homework for the next month will no longer be granted."
The second-year class ended, and the students from Gryffindor and Hufflepuff left the classroom excitedly.
Theoretically speaking, this class was their first Defence Against the Dark Arts class in the true sense. Last semester, Professor Quirrell's defense classes, where he read directly from the textbook every day, were no different from self-study.
Professor Cavendish's class was very interesting.
This was the consensus among the first- and second-year students who had already attended the Defence Against the Dark Arts class on the first day of school.
Moreover, the matter with Tom allowed them to realize something from another perspective. Professor Cavendish, who usually wore a cold expression and spoke with a harsh tone, actually didn't seem as distant and unapproachable as he appeared on the surface.
A person with a cold heart certainly wouldn't like Tom and Jerry, a cartoon, would they?
Thus, even though Sherlock acted like Snape—maintaining a cold face while speaking and teaching—the students, especially those who had already attended the defence class, weren't particularly afraid of him.
Just a few days after school started, several outgoing Ravenclaw students had already gone to Sherlock's office to ask questions related to Defence Against the Dark Arts.
Judging by this trend, the interaction between the Hogwarts students and the new professor was very friendly. This was also what Sherlock was happy to see.
Only when the students realized that his inner self wasn't cold, solitary, and problematic would such a view influence the other professors at Hogwarts. This would subtly complete the change in his image in the minds of Professor McGonagall and Dumbledore.
Just as Dumbledore had said at the start-of-term feast, he looked forward to Sherlock's change, so that he could openly show that change to him.
The teaching for the first and second years ended on the first day of school. Regarding the upcoming Defence Against the Dark Arts classes for the higher grades, Sherlock was even more prepared.
On the morning of the second day of school, while the fifth-year students were whispering in anticipation of what kind of unique class Professor Cavendish would start for them, they saw Sherlock walk in carrying a thick stack of parchment.
"Now that you've entered the fifth year, do you know what the biggest challenge you will face is?"
At the very beginning of the class, Sherlock didn't waste words and went straight to his opening remarks.
Below the podium, the group of students all shook their heads with confused expressions; they weren't sure what kind of challenge they were about to face.
Sherlock shook his head in disappointment.
"Each generation is truly worse than the last. When I was in school, even fourth-year students had already started to get nervous, yet you've all entered your fifth year and still don't know what you should be preparing for."
These words were entirely Sherlock borrowing phrases from teachers in his previous life and applying them to Hogwarts as empty rhetoric; he hadn't even attended Hogwarts, so how would he know whether the Original Owner had been nervous in his fourth year.
"O.W.L.s—Ordinary Wizarding Level Examinations. At the end of the fifth year, you will face the first magical examination that will determine your future."
"Unlike previous exams, the O.W.L.s will be organized by the Ministry of Magic. A total of twelve subjects will be tested. The grades will still be categorized as O, E, A, P, and D, but these results will determine the Ministry's assessment of what you've learned in five years at Hogwarts, and they will also serve as an important basis for other wizards to judge your abilities after you leave school."
"Defence Against the Dark Arts is one of these twelve subjects, and it is mandatory for everyone."
"Professor Quirrell's teaching last semester was not ideal, which means you have actually fallen behind compared to the normal yearly learning progress. Therefore, in the defence classes over the next year, you must work doubly hard to cope with this examination."
"For your studies in the coming year, I have arranged a complete plan—the Exam-Oriented Education Plan."