The assembly is for more the leavers rather than for one leaver so that leads to Leavers' Assembly. (I know there is a different view in some cases- eg Patient's Charter/Passenger's Charter but there is a specific reason in those cases because the writers wanted to imply a personal contract.)
Some may argue that no apostrophe is needed at all because the leavers don't possess the assembly - the assembly is for them
Singular is apostrophe s.
If the plural ends in s , then it is s apostrophe.
Men, women , children and other nouns that do not end in s when plural , it is apostrophe s
I'm clear on all the rules apart from the accepted (but not to me) practice of placing an apostrophe after the S for a name that ends in S- eg Mr Jones' car (why not Mr Jones's car), Achilles' heel, and of course Pythagoras' theorem (why not Pythagoras's theorem?)
And I've always wondered where the apostrophe goes on possessives of plurals that end in X- eg gateaux