Khan Academy is another great tool for both students and teachers that makes it simple to learn or review multiple subjects. This website specifically can be a great resource for studying mathematics with their detailed video lessons and tutorials.
Freckle Math
Freckle Math is a resource that many school districts use that correlate with their mathematics curriculum. It is very interactive for students, including practice problems and competitive games. It is a great online material for virtual learning to keep students at grade level practice.