It is possible for students to read high school level textbooks online. Online textbooks usually cost a fee, but free ones exist and usually come in a PDF format.
Popular textbook websites, such as Pearson SuccessNet and McDougal Little Classzone, often work in partnership with schools to give students access to online textbooks. Schools pay for these textbooks, and students can access them with a given code. Some, like Pearson My Lab & Mastering, have activities and quizzes that complement the textbook material. Free high school textbooks do not require access codes and students can access them through online directories. Open Culture and TextbooksFree.org are two examples of websites that host PDF files of various textbooks.