There are several explanations:
1. Teachers and students are in the different courses:
E.g. The teacher is in elementary school course whereas the student chooses to be in junior high school course. In light of the situation, student will not be able to join the class by entering the class code.
2. Students enter the code in the wrong field:
The Class Code needs to be entered in "My Classes"→ "My Classes" on the upper Game Map. If you enter the code in the PaCode field on homepage, you won't be able to join the class.
3. Wrong alphabets and numbers for Class Code:
Note if there is an additional space before or after the code.
4. The assigned Seat Number does not exist or is used by other students:
If a Seat Number is used by other student, next student who enter the same Seat Number will not be able to join the class. The teacher needs to revise the Seat Number of the occupier from "Teaching Console" → "Manage Students" and then invite the student to join again.
If the teacher does not choose "Auto Assign Seat Number" when establishing a class, then students don't need to enter seat numbers to join.
5. Class is full:
If the class is full, students cannot join. The teacher has to click on "Teaching Console"→ "My Classes"→"Add" to increase the number of students allowed in this class.