Your argument is similar to burning the house down, once you discover that you don't like the couch in the living room. Or, more realistically, arguing against taxation based on the idea that rich people avoid being taxed anyways, and it's only poor people who will get the short end of the stick. The school system isn't perfect, and is hard to improve due to many reasons, but this doesn't mean we shouldn't try. It has a purpose which is much more important than the suffering of any individual who goes through it. It's a shared good that can only be made better if everyone participates. When people who can contribute the most are allowed to be excluded, the whole thing becomes worthless. But, guess what, those who thought that they may be exempt from contributing to the public pool will inevitably find out that the public who was in this way deprived of a public good hates them, and will eventually come after them with pitchforks and torches.

Learning how to co-exist with different sorts of people is definitely a valuable experience.

Trying to do that in an completely artificial institution that arbitrarily divides people into age cohorts in a way that resembles no organic social pattern and forces all social interaction to conform to bureaucratic rules is not just not a value experience, but in fact actively inhibits the above goal.

The kinds of social skills and expectations kids develop in a school environment often need to be unlearned entirely in order to function effectively in a complex and dynamic society.