Agenda was originally the plural of Latin "agendum" (thing to be done) and is still considered by some to be a plural form.
In modern English; however, agenda is usually considered as the singular form with agendas the plural: What is on the agenda for the meeting?
The secretary kept track of the two executives’ agendas.
After his secretary printed the new agenda, Gordan realized he also needed the revised addenda which should be printed in an addendum.
So well established is agenda as a singular that agendas is now commonly heard and seen as the plural form; however, the correct Latin singular is agendum; as, "The agendum is whether we want an extended school year or not."
"The agenda have been established for the meeting."
You have the choice to follow hoi polloi or to write it "correctly".