Both sentences can be correct, but they have slightly different implications:
1. "My daughter goes to boarding school."
* This phrase often implies that she is attending a specific type of school where students live on campus. It's a more general statement about her education.
2. "My daughter goes to a boarding school."
* This phrasing specifies that she attends one of many possible boarding schools. It suggests that there are various options for boarding schools, and you are referring to one of them.
So, you can use either, depending on whether you want to make a general statement or refer to a specific type of institution.