This site collects almost a thousand textbooks used in Chinese schools, ranging from grade 1 up to senior high school. Subjects cover Chinese, mathematics, English, physics, chemistry, biology, history, geography, politics and science. There are many different ways to use these textbooks in your own studying, and I've written an article about this specific website here:
https://www.hackingchinese.com/866-digital-textbooks-for-expanding-your-chinese/