How large were the classes? Did each class receive only 1 of these teaching methods?Two different teaching method were used on these classes.

Is your intention to compare the 2 teaching methods, so that you could determine which is perhaps better?In order to compare their academic performance under the various teaching methods, what should I do?

No, AFAICS.a) As all scores of two classes are well collected, could I treat I got the population mean, population variance….

With kind regards

K.