@McLXIX
Have you heard of the Lori Loughlin USC scandal going on in the states?? Parents paying to get their kids in prestigious colleges. Not just dumb kids like those. You hear of Chinese parents paying millions just to get their kids into Harvard. What I am trying to say, you can have well-to-do parents that you think should know how to do the "right-way" to raise kids, or provide them with proper tutoring or just raise them themselves and watch over them enough to make sure they are doing ok. But examples above sound unbelievable over fiction but it happens that you have dumb parents that let their kids slip through the cracks.
In this case, I will just make up a scenario where the step father is a nice guy, but he doesn't know how to be a great parent. He cared for a dying mother of 5 and as she dies, wants to marry her and care for her kids after death. Pretty tough for a young doctor to inherit 5 kids at that age and know what to do. And sounds like busy with his profession to be that loving and maybe not know how to raise. He sees them relatively happy, provide for them and they get by in school and he's thinking they are ok. If Yot's didn't fail the supplementary they probably would have kept going and getting by and cheating on tests.
As far as Fuu, when the father finally did realize after the quints had to transfer schools then he probably realized he needed to do more. But he met Fuu's dad and somehow got to know him to convince him to let his son tutor. It's not that huge of a stretch... if you hear the boy is the top of his class every year, then why not have a person closer to their age try to reach the girls and tutor.
Yes Fuu came off like an a$$ lol to Itsuki. But really, only Nino hated him for her own reasons, Itsuki didn't want him to tutor her but she got along with his sister and is empathetic to his debt situation, and Miku didn't hate him but needed winning over. Ichika and Yotsuba at no time hated him. And Fuu wasn't gonna give up because frankly he needed the money and didn't want to let his sister down by losing the job.
And as I mentioned before, if Yotsuba remembered Fuu from the beginning, then maybe the step dad also remembered Fuu from 5 years ago and he had another reason to unite him with the quints. Again he coincidentally got them together in Scrambled eggs and also made sure the quints and Fuu were in the same class at the new year. There is more to this story so stay tuned.
But your right... if Yotsuba did go off to another school, she may have been just fine and developed a better sense of self. But it would have cost her the bond she has with her sisters and it was the sisters that weren't going to let it happen... not the father. Not after they all admitted to cheating to pass the supplemental lol.