It was already expected that the writing was going to be really lazy and loose but it's still quite hard to read... things that don't work are just forced onto you and things that should be shown are just avoided at all cost, for good reason since it would destroy the entire plot. Then there's the forced "shounen protag" moments where nothing has been done for it to happen or rewarded for, like low hanging fruit but the fruit isn't even hanging, it's just on the floor. It feels like the writing is some times fighting itself... at one point it's time travel to do things more efficiently, in another it's "Idk what is right for me to achieve my goals", then next it's "I want to adventure!" and know for sure that's the path over whatever-it-was-in-the-beginning, and last is to proceed to adventure learning that "this is inefficient", proceeds to not mentioning that they need to change/improve and cue in the artificial romance that has not even been built up. What IS this?? 😩😩😩 Also, how is this happening now and it doesn't seem like the MC knows about the girl at all?? It's time reversal, isn't things as it was?? If the girl was going to transfer in then it should have been in his memories with any particularities about her in mind... the entire story is playing out as if he has no clue about what's to happen at all!!
...Did you forget he's a master magician? He just can't use his magic to its greatest potential due to his weaker body as a child, but being able to use his magic is not an unearned power jump in any sense. None of your
at one point it's time travel to do things more efficiently, in another it's "Idk what is right for me to achieve my goals", then next it's "I want to adventure!
Complaints are reasonable as those are not mutually exclusive and, more importantly, he is trying to do things in a different way than he did before. In this case, he had wanted to adventure but had instead chosen to isolate himself and focus on studying the first time.
As for the girl, there was no indication that he didn't know she'd be in his class (to say nothing of the fact he was an old man when he returned, even after a couple decades you'd have forgotten a lot of people from your childhood). He didn't interact with her, they weren't a thing the first time. Simple as that.
The romance thing? Hasn't even been hinted at and she's a child acting like a child. Simple as that.
I don't get what's hard for you to understand about this story. It seems like your complaints are that he's doing things, according to him, differently than his first life.
Granted, doing things differently is clearly being used as an excuse to make an adventure story, but hey that's what it says and so it's still different.