Many good explanations here. Well done Baggy! I confess to being a retired teacher too and would add
Point 1 - apostrophes NOT apostrophies.
Point 2 - 'ignorant about its usage' NOT it's usage.
Point 3 - DON'T use it just because a word ends in 's'. Usually this just means a plural - boys, apples, threads, grans, opinions, apostrophes,sons, daughters.
Point 4 - DO use it to show possession. My daughter's baby. My son's partner.
For 1 daughter or 1 son these would be 's.
If I have 2 or more daughters and sons it would be My daughters' babies or My sons' partners. These it would be s'.
I'd say of all the mistakes we are likely to make, the most irritating one would be the one at Point 3.
Finally, I agree, if in any doubt MISS IT OUT.
Heather, sorry but you are wrong. There is a final 's' for different reasons. Sometimes it needs an apostrophe, sometimes it doesn't. If we stuck to your suggestion then we'd all be making the mistakes discussed in Point 3. Heather's 40th is correct! (possessive) Heather's 40 is correct (Heather is 40)
I have seen young teachers make these mistakes but the rules have NOT changed.