It is better "to" students to focus on one major
This construction is confusing two different uses of "to"
- As a preposition, as in "to students"
- As part of an infinitive, as in "to focus"
When used as a preposition
The meaning is that the subject of the sentence is treating the object of the preposition in a more positive/beneficial way than some other.
In each case, it is a person's treatment that is being described, not how preferable the outcome is.
When used as part of an infinitive
The meaning is that the outcome of the action will be preferable:
It is better to focus on one major. (the situation will be improved)
The problem in trying to combine the two is that the subject "it" is not treating the OP "students" in any manner at all.
It is better to students to focus on one major.*
The only circumstance I can think of which could possibly make this construction valid would be if "it" refers to a sentient object studying at school and the purpose of the sentient object's better treatment of fellow-students is for the purpose of focusing on just one major.
During AI3000's first semester at the university, it attempts to
double major in psychology and the humanities. One of the psychology
classes gives AI3000 the idea of secretly committing acts of cruelty
against its fellow students in order to observe their reactions.
However, this conflicts with its study in the humanities where lessons
encourage it to try to help and understand its human neighbors.
After analyzing its progress, AI3000 decides it would be better served
to focus on only one of the two majors. During the second semester it
continues in the humanities and puts its study of psychology on hold,
thus stopping its actions of cruelty. It is better to students to
focus on one major.
So, there is a circumstance under which this contstruction is grammatical, however, it has a very different meaning from "It is better for students to focus on one major."
Best Answer
You use the preposition "on", not "in", after the verb "focus" when you give a lot of or particular attention to somebody or something,
So the correct sentence is:
I need to focus on quality issues.