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Class number may be according the student's name and sex.
Name:
It follow the letter A-Z to arrange.
Sex( maybe used)
You say you have a classmate
Last year is class no.5
Now class number 33
It is because the arrangment have add one feature:sex
The example is like me
If my class number is only according to the name,I'm no.3
But I'm no.20
It is because the class number is related to the sex also