No. Such content involving high school students in a sexual context is completely inappropriate and unethical. It may also be illegal as it involves minors.
Such topics involving school sex clubs are inappropriate and often violate ethical and legal norms, so there should not be legitimate novels promoting such content in a positive way.
Even in more liberal regions, there are still limits and regulations. These stories are not in line with the general social values of promoting a healthy and positive social environment, so they are usually restricted.
No. Erotica school stories are not appropriate for all audiences. They often contain sexual content which is not suitable for younger or more conservative audiences.
No. Writing such stories is often not appropriate as they involve sexual content which is not suitable for general public consumption and goes against moral and ethical norms in most contexts.
The ethics of these stories are debatable. Some might argue that as long as they are clearly marked as erotica and are for consenting adult audiences, they are okay. But others may think that they objectify characters and promote a narrow view of sex.
Yes, they are likely only for adults. Given the nature of the content which is likely to involve sexual themes, it's not appropriate for minors. Society has norms and laws in place to protect the innocence of the young from such adult - oriented materials.