is adding the comma before was necessary?
a project involving training for children, teachers, and parents(,) was the most effective
is adding the comma before was necessary?
a project involving training for children, teachers, and parents(,) was the most effective
My gut tells me to leave it out as you are neither adding a new item (I.e. commata to enumerate different entities) nor are you closing a parenthesis.
Similarly, if the enumerarion of persons is removed, it reads fine for me:
...a project was the most effective.