Over 100 years old is a bit of a stretch, the foundation of modern quantum physics was laid mostly in the 1920s...so "nearly 100 years old" might be better.

Realistically though, the reason classical physics is the basic physical foundation laid for most students is simply that it is tremendously easier to understand and calculate and is basically "correct" for 99.999% of things people encounter in their real lives. Schools already teach far too many things which are somewhat useless later in life, why should most high school students be subjected to quantum mechanics when they don't even have the mathematical underpinnings to even come close to really understanding it.

In 100 years or so when we have the math and processing power to solve a five or six body gravity equation then maybe it can actually be taught to those who don't specialize in it, until then the classic approximation is pretty good for high school work.