What is a good thesis statement for an argumentative essay on public schools vs private schools?
You can't write your thesis statement until you decide what position you are going to argue for or against in your essay.
If you want to argue that public schools give students a more real-life experience because students in a public school will be more diverse, your thesis statement might read, "Public schools provide a more diverse experience than do private schools, thereby providing a better education for their students."
If you want to argue that private schools are superior because they can provide specialized instruction to meet the needs of smaller, more narrowly identified groups of students, your thesis statement could read, "Private schools are designed to meet the needs of students with specifically identified educational requirements better than are public schools."
Once you determine the argument you want to present in your essay, you can develop your thesis statement.