Once an activity has been run, it still needs to be evaluated. This gives the opportunity to both show off how it went and to learn from mistakes made.
Evaluation takes two main forms, which can be done together -
EXTERNAL is basically to show to other organisations, e.g. funders to prove some point or other.
INTERNAL - is done so you can know how things went and what to improve.
Evaluation is only as good as the use you make of it. External evaluation has to be presented to the right people in the right way. For a school this can mean parents, school - whoever you want to like you. Internal evaluation has to be used as an opportunity to learn.
Events aren't finished until they are evaluated. Everybody should evaluate.