If you've forgotten your password, you have a few options.
Live Password Reset
You can send a request to your Teacher to give you access to reset your password. To go this, first enter your Class Code from the student login page at khmath.com/play
Select your name from the list. You don't need to remember any usernames.
Press the Forgot Password link.
This will send a request to your Teacher (if you're logged on). They will see a green bar across the top of their screen with your request.
Go ask them in person to approve it. Once they do, you will be able to enter a new password.
Remember your Class Code
This is why it's crucial that you write down your Class Code so you can easily enter the class later.
Reset by E-mail
If you had already added an e-mail address to your account, or used Google to sign in, you can reset your password by going to the normal Login page for non-Students at khmath.com, and press Forgot Password. You can then enter your e-mail address and it will e-mail you a link to reset.