The university of Tokyo provides several free Japanese courses for beginners / intermediate / advanced level Japanese for international students.  Apart from this, there are some useful websites that can help you learn Japanese.
Happylilac is a popular website that provides several lessons on Japanese school subjects, you can choose “国語”(kokugo) to access lessons for Japanese from primary school level to high school level.

Additionally, there are other subjects like science, math etc., that have lessons in Japanese. So, if you are interested in learning a specific subject in Japanese, it is the go-to place!
Tobira is also another website that I used during my initial days in Japan. It has many free videos, learning materials for beginners.