You can recover the username and password from the “Forgot password?” link on www.10monkeys.com.
Please note that the username for the Main User account is school specific, meaning that all teachers in the same school will use the same username. The password is different between the teachers. If a teacher changes the username, it is his or her responsibility to inform the other teachers at the school about the new username.
If recovering the username and password does not help, you can order new login details to Math World from here. Please provide us with your school name (including city/county and country).
As a teacher you can change the password for a student in the User Management area, which you can access through the link on the right sidebar in the game. Please note that you don't have to know the previous password in order to reset it.