Honestly he wasn't half as dumb as he let her believe. Between Ron who hated everyone better than him and Hermione who was more competitive than an Olympic sprinter, Harry made sure to keep himself average if for no reason other than to avoid the two irritating him. Harry was sick of having to act for the sake of others.
Growing up with the Dursley's Harry had learnt to act a certain way and behave a certain way, he had to learn quickly if he wanted to survive that hellhole. During Harrys first few years there he was more than willing to push back if someone pushed him first, he talked back if someone said something to him and he acted like he wanted. But that quickly disappeared thanks to constantly getting beat up by his cousin and his friends along with his uncle's over use of his belt and his aunt's love of denying him food.
He was also a lot smarter than he let on, but he had to dumb himself down because the last time he dared to do better than Dudley on at test he got locked in a cupboard after an hour with Uncle Vernon's belt. It turned out to be good practise for when he came to Hogwarts as he already knew how to dumb himself down for others. Average grades made sure that Ron didn't get too angry with him and made sure he didn't have to constantly suffer thanks to Hermione.
Eventually Harry learnt to change the way he acted and spent years acting meek and week just to get through life with minimal suffering. However the important thing was the fact that that Harry, the original, never left. He was merely pushed back so meek Harry could take over. However Harry knew he was still there, he could feel him, sometimes even let him take control.
In his first year when Quirrell tried to strangle him and ended up burning himself, Harry had frozen in fear, that was until the real Harry took over and finished Quirrell off by grabbing him and burning him to death. The same with the Basilisk, the dementors, it was the original Harry who had taken care of the problem, not the fake weakling that he had to pretend to be.
After nearly an hour of Harry sitting in silence he came to a realisation, he no longer had to pretend to be someone he was not. The purpose of weak Harry was to minimise his suffering when he could not fight back, but now...now he could. Here he was armed the same as everyone else, except he had an edge that they did not. He was the one who had killed at the age of eleven, he fought Slytherin's monster, he fought off an army of soul sucking demons. They had not seen what he had seen or felt what he had felt.
Each near death, each and every moment of suffering helped him, made him stronger, even in simple ways like at least increasing the amount of pain he could endure. He had the potential to become great, his 'friends' had abandoned him. This was an opportunity, no longer would he spend years pretending to be someone he was not and no longer would he have to put with people he did not like, no longer would he be someone he was not.
"No more." Harry whispered to himself as he closed his eyes. "No more pretending, no more, now...it's time for the real Harry to come out." Harry said, as he opened his eyes. If he had a mirror then he might have noticed the brief glow that came from them.
"The great Harry Potter sir?" A voice interrupted his silence. Harry's wand was out in record time and aimed at the intruder before they could even finish saying his name. "Don't shoot!" The intruder cried and put his hands up in a submissive gesture.
"Dobby?" Harry sighed, rolling his eyes at the house elf in front of him. "What the fuck are you doing here?"
"Dobby was cleaning great Harry Potter sir, then Dobby finds the great Harry Potter sitting on the floor." Dobby replied, looking at him and waiting for him to answer the unasked question of why he was sitting on the floor.
"Yeah well...I'm not doing too good right now Dobby." Harry confessed, lowering his wand.
"Oh no!" Dobby replied as if he was just told the most horrific news in the world. "Can Dobby be doing anything to help?"
"That depends," Harry said after a moment of thinking. "you can go anywhere in the castle right?"
"Yes Harry Potter sir!" Dobby nodded quickly, eager to help.
"I want you to go to the Gryffindor common room, get all of my things and then come to me when I call you, can you do that?" Harry asked.
"Oh yes!" Dobby nodded so fast that Harry suspected the elf had hurt his neck. "Dobby can do this!"
"Fine, go get my stuff and wait for me to call you." Harry replied as he got up to his feet. "I'll be off, wait for my call." Harry said before he walked out of the classroom and down the halls.