You're right. I forgot to mention that the only time one would use an apostrophe is to form the plural of a single letter.Here's a case where an apostrophe, though illogical, might be justified:
There are two e's in the word "element".
It is a very widespread mistake, but your teachers are correct.In the 30s = Negli anni 30
30's vintage dress = Vestito degli anni 30
This is what I learned in my grammar school. Then I saw so many examples of 30's, 50's, CD's used as plural that I didn't know anymore, so I asked "qualified" teachers and all of them told me that, grammatically speaking, is wrong, that it should be 30s, 50s, CDs. Unfortunately, the use of 's to make a plural is so widespread nowadays that nobody knows anymore