It's extremely hard to find teachers that are willing and able to produce entire curriculums on their own for the group of 30 children. However, there are many teachers that would be excited to try out better curriculums if they were offered them. That curriculum can, of course, be less structured as well.
Hundred percent agree on lack of resources, of course.
My state provides about $7,000 per public school student (I don't know if this is per student actually enrolled or student in the district, even if the student goes to a private school). That means that a classroom of 30 students is worth $210,000. There is absolutely no way that $210,000 is an insufficient amount of resources to educate 30 K-12 students. That's 3½ times U.S. median income — easily enough to teach a year's worth of material.
The problem with public schooling is structural, not resources.
Arts classes requires materials. Instruments for band. Mopping the classrooms. Maintenance for desks.
Some of those kids go to detention, so now some teachers get paid overtime to sit with them.
The football team takes the bus to go somewhere.
The library buys some new books. Updates the computers.
There's a lot of stuff going on, it's not just the teacher.
You mention "public school" in the previous sentence, which is important, because many private school do successfully address the engagement issue by having a much more rich and unstructured environment with better teacher to student ratios and passionate teachers that more freedom to innovate. That doesn't mean that there aren't great teachers at public schools, but that the standardization and lack of resources really hobbles them.