Tetsuya was given this team since all of them were quite familiar with him. And since Yuzume was alive, they didn't need Kakashi to teach Sasuke how to use his sharingan.
As for the reason why Sakura was replaced by Karin, the reason why Sakura was placed in team 7 in the first place was mostly because Naruto had a crush on her since she could be a good bond for Naruto.
But now, Karin added more value to the team due to her being able to activate the Adamantine Sealing Chains and being reasonably proficient in Fuinjutsu while still being a Tracking Ninja and Medical Ninja at the same time.
And also the fact that she was already close friends with the two other team members.
Kakashi was also given team 8 instead of Kurenai since she was a newly promoted jonin.
Sakura was sent to a team of civilian to balance the teams since she was one of the best in the academy, on paper.
Well, sorry for you Sakura. I'll still recommend you to Tsunade if she becomes Hokage.
…
At the academy…
The team compositions were soon announced.
Team 7 was quite happy with their team composition since all of them were close friends. Even the usually broody Sasuke smirked.
Soon enough, Tetsuya came to pick them up. The three of them were quite happy but also quite surprised to see that Tetsuya was their Jonin sensei.
Tetsuya never told them since he wanted to keep it a surprise.
They soon headed to the roof and Tetsuya started to talk.
"Since we all know each other, there's no need for introductions. While most jonin sensei test their students before accepting them, you guys are all kids I taught so I know your level quite well."
As you guys already know, the chunin exam will be in July so if you want to participate in it, you'll have to do so as a team. So try your best to improve your teamwork as much as possible during these six months if you want to participate."
"Yes, sensei" all of them said in unison while being quite excited about the prospect of participating in the chunin exam.
"Well then, let's get our first mission."
As they arrived to the mission center, the third hokage soon welcomed them in.
Tetsuya decided to take a C-rank bandit extermination mission.
The Hokage looked at him doubtfully but after he told him that he was planning on letting them enter the chunin exams so they needed to adjust to ninja life as soon as possible, the Hokage reluctantly accepted.
…
After letting them prepare supplies, they finally departed. He decided to go in the afternoon like during the time of his first bandit extermination.
Naruto was quite excited since it was his first time out of the village but his enthusiasm was soon knocked down a notch after he realized that they were going to kill people.
"Well then, while I said that I won't test you guys for entering my team, this is a test to see if you really have what it takes to be a ninja.
You may think that ninjas are cool heroes saving people here and there, but ninja are mostly just murderers for hire. So if you can't handle that, I recommend you return back to village as a civilian."
All of them were silent after hearing this but after a few moments, they made their mind and rushed towards the bandit camp.
The mission Tetsuya chose deliberately didn't have any hostage to rescue since he wanted them to focus on exterminating the bandits.
Tetsuya observed them to make sure that they get at least a kill each since the point of this mission was exactly that.
20 minutes later, the three of them finally came back with sullen faces. After confirming that no one else was alive, Tetsuya finally started talking:
"Good job on your first mission. During the next six months, we'll be doing a lot of missions like this since becoming a chunin will require you to do a lot more killings like these.
So I suggest that during this night of rest, you make up your mind about whether or not to continue being a ninja and turn in your resignation letter once you come back to Konoha."
As the morning came, they soon went to the employer to report and went back to konoha.
After they arrived at the gates, Tetsuya soon asked them." So, do any of you guys want to quit Ninja life?"
Naruto responded "I never give up because that's my Ninja way. As long as we don't kill innocent people, I won't give up being a Ninja"
Sasuke:"As long as I can kill that man, I'll do anything."
"Wherever Tetsuya-sensei is, I'll be there" Karin responded with a perverted smile on her face
Tetsuya just smiled and soon headed to the mission center to report the mission.
After they went to report to the mission center, they headed to a BBQ restaurant to celebrate the official formation of team 7.
"Congratulations on finally becoming true ninjas" Tetsuya cheered while shoving meat in his mouth but the others weren't in the mood to eat meat after what they saw.
"Well then, our schedule for the next six months will basically be at a D-rank mission every day until you guys reach 50 missions and at least 1 C-rank mission per week. Also I'll start teaching you guys some jutsus after I test your chakra Nature."