Most schools, especially elementary schools, will have music classes for kids where they get to learn to sing, read music and maybe even play an instrument. But why are private lessons so much better for students.
Customization. It is completely tailored to them. With groups, teachers have to follow an agreed upon curriculum and go at the pace of the slowest learner. Private lessons 100% follow your child and their needs. There is no frustration at having to wait for a slower learner, nor at being the slower learner, as progress occurs at their own rate. And the curriculum is chosen to fit them and can easily be modified.
Strong Foundations. Especially with large groups, bad habits can easily be missed. With one-on-one instruction, you know they will pick up strong habits from the get go. It's easier to correct basics the first time, rather than years later when it might be too late. Plus, more music theory tends to be covered as it is easier to add in those concepts with one student.
More Momentum. In private lessons, they will progress faster than they would in group lessons. They have more time in the lesson to play with and for the teacher, plus the structure of a private lesson makes at-home practice more compelling. (It's harder to fake playing when you're the only student). Faster progress usually means more enjoyment and less chance of stagnating.
More Options. After taking private lessons for a few years, it is much easier to join any kind of musical group, whether it's the school orchestra, or they're starting their own rock band. Or they might have the option to complete a music exam, like RCM. Even switching into group lessons after a year of private lessons often lets them join a higher level of group.
Easier scheduling. This is a more logistical reason, but it is much easier to arrange or change the time of a private lesson. It only involves 2 people's schedules, versus trying to find a group lesson, of the appropriate skill level, with good classmates, and a good teacher, at a time that works for you.
This is not to say that there is no place for group lessons, as they can be great for their own reasons. But it is important to weigh the pros and cons in terms of your own child when choosing what type of lesson to place them in.