For the Algebra Challenge, mastery is reached when a student is able to answer three questions similar to the format below with no errors and with efficiency (no extra steps). The examples of equations posed vary.
The examples below depict the approximate range:
a*x + b = e + d
(a*b*x)/b + e = d + y*z
It is likely a student will reach mastery before the final chapters of the game. There is advanced mastery as well in later levels but that advanced mastery metric is not the target of the challenge.
On the leader board, the classroom percentage is determined by "Of the players who have logged in, how many of them have reached mastery". A 90% means that 90% of the students in your class have reached mastery as defined above. A likely misconception is that 90% means that your students understand 90% of the concepts - we do not measure partial mastery.