If you are a teacher creating an account for the first time, you will need your school's code. To acquire your school's code, contact Chandra Horvath at firstname.lastname@example.org. A student gets all codes from the teacher (most teachers have multiple groups based on period or level of the class, etc). The first time he will join with a teacher code, then click "join group" to keep adding the codes for different classes.
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Teachers, you only need to enter this school code once. Students do not need a school code to sign up.