Inserting possessive apostrophe

VoiceIn sometimes (but not always) adds the possessive apostrophe when the noun is singular (“a student’s grades”) but doesn’t ever seem to insert it when the noun is plural (“first-year students’ grades”). I tried adding a custom voice command to add the apostrophe, but when I use the command, it inserts the punctuation mark with spaces around it (“first-year students ’ grades”). Is there any way to automate possessive apostrophes more effectively?

What voice command did you add?

I tried students => students’ . It works fine.