The help you get from tutoring actually depends a lot on school teachers. If your school teacher is competent enough, they will explain everything and you won't need any assistance outside of class, but unfortunately not all schools have good teachers, and most (in my experience) teachers are either lazy or are not up to the job because even some hard questions they cannot answer themselves, despite the fact that they have been teaching for many years, and that's where outside tutors comes in, to help you get knowledge that you will, or maybe miss in class.
Also if the teachers are smart themselves, but not able to explain the concepts in a simple manner that everyone in the class is able to understand, that's where a second teacher/tutor is needed, and the question about learning in classes or 1 on 1 depends on the level of attention needed by the student to grasp the concepts.
Another side to the question is whether the student is commited to work, if the student does not concentrate in class, even if the teacher or tutor is great in teaching and smart, you won't get any out of it because you didn't get 100% of what is said by the teacher/tutor, and of course, if teachers repeat the stuff often, twice or three times, which helps student to grasp it, but if students concentrate enough and got it in the first time, then no need for any second time or going home to revise.
Personally, I don't think "abilities" of students affect the students that much in learning, if you know enough, we humans are 99% similar in DNA, what makes us so different in the so-called "abilities"? Does only 1% makes such a big difference?
What makes the real difference is the amount of effort the student puts into the course, and the foundations the student laid in the basics, which also relates to the effort the student made in beginning years.
Why did I say so? Well, if the student didn't grasp the basics so well, then of course they will struggle with getting the later on topics because it derives from the basics, and since classes flows on and on, if the student builds up all the points they didn't understand, that's what causes the different standards in the class.
So what a 1 on 1 tutor can do in this case is to revise over the basics and links all the links between the basic stuff and the advance and that is the only method to raise a student's mark, under the assumption that the student is willing to learn about it.
Hope that answers the question.