It's a trick question. He's two faces, just at different times of the day.
At the start of the day, the Professor is

.
I'll proceed by straighforward deduction, and we'll see who the professor is at the end of the day.
The class was about relativity that day. The Professor gave a quiz. Mr. Bright

and Mr. Smart

were well prepared and ready to go. On the other hand, Mr. Sleepy :zzz: and Mr. Surprised didn't know there was a quiz, because they hadn't been paying attention when the Professor

annouced it.
Mr. Funny

thought it would be a hoot if Mr. Gross started making bodily noises during the quiz, and he talked him into it. This caused waves of laughter, and the Professor became visibly upset. But it soon died down, and the quiz ended.
Then the professor

collected the homework. Mr. Jock

and Mr. Mean

didn't do theirs, so they tried to copy Mr. Nerdy's and Mr. Geeky's homework. But Nerdy and Geeky wouldn't let them, so after class

and

gave Mr. Nerdy a wedgie

, and they gave Mr. Geeky a swirly

.
During lab, Mr. Flirty

and Mr. Horny

were more interested in the girls at the next table (there are only 2 girls in this class), and started making eyes at them. Ms. Shy :shy: and Ms. Bashful

noticed, and found it difficult to concentrate.
Mr. Bright

and Mr. Smart

were getting good results, but the rest of the class was having trouble. Mr. Confused

and Mr. Slow

weren't able to put together the setup. Meanwhile at another table, Mr. Silly

had his lab manual opened to the wrong page, so he and his partner Mr. Grumpy wasted the whole hour. Mr. Sad

and Mr. Dumps

broke their apparatus. Finally, Mr. Ramkiesel and Mr. Geist1024 were a pair of crackpots (damn, I'm not just clever, I'm also subtle) and they took the course just to get in the lab and prove their theories correct. But when the data came into support relativity, Ramkiesel couldn't believe his eyes

. But Geist1024 was unfazed. He balked at the experiment

, knowing that it could be trumped by a sufficiently clever
gedanken experiment.
So it's elementary my dear Watson. At the end of the day, the Professor wasn't

anymore.
He was

.