Hello! Next year I will graduate my school in Kazakhstan and I want to apply for American school as an exchange student for 12th grade after that. I'm gonna be 16 years old. And I have a host family that will accept me. What should I do to apply for school in Pennsylvania? And is it possible?

Only the school district you will be attending would be able to answer this question. Ask your host family.

If you have a host family, they are in a school district and should have asked the school these questions.

In the US all school districts are local - the schools in each town operate with different rules and regs as mandated by their school board elected by the people living in the district.

If you want to attend the regular public high school your host family would have to arrange that directly with the school as it is your residence in their house that would allow you to attend.

Have you looked into the necessary visa that would allow you to stay in the US for that long? (Tourist visas are for 90 days only so you would need a visa as a student for the school term and I'm sure your host family - or the organization which has linked you to the family - would need to provide info that the school has accepted you.)

Yes, to get a visa for academic year I must be accepted to the school.

Pennsylvania is not a small place - where does your host family live?