"Mae ffrog briodas Alys yn brydferth."
Translation:Alys's wedding dress is beautiful.
I was taught to simply put an apostrophe after the s tu pluralise a word ending in s in English. This should be accepted AT LEAST as an alternative correct answer!
I assume you mean, you were taught to put an apostrophe after an "s" to signify possessive, since there is no plural in this sentence. In which case yes that should be an acceptable answer, I'll add it to the database now if we don't have it already.
Current usage, according to Longman's Student Grammar, is to use the form given. Also Wales's, Jones's, etc.
When you read it out loud, if you say two s-es, then you should write two s-es, as in Alys's.
That's a rather odd school, EV! What is the class's reaction when taught they should write "the class' reaction"? ;)