Yeah Mathster is an online assessment platform dedicated to helping students at any level achieve their best in the subject. 

AMSI is a teacher-oriented site. However, students can use it to improve their grades by studying the curriculum beforehand. 

Mathsisfun is an online website with various high-precision mathematical tools (E.g. Calculators) and complex categories such as algebra and geometry.  The difficulty of these problems arranges from Kindergarten to Year 12 level maths to assist students of all ages.

Learning Potential is an official government website that helps students in trouble with English schoolwork and push them to their full potential.  (High school, age 12 ~ 18)

Common Sense Education provides a list of helpful science websites that you can access to prepare for school assessments.