Sarah and Jimmy's After-School Carnival is an after school event that Sarah and Jimmy both take part in after school is over. The event consists of Jimmy up on a moving platform, shaped like a unicorn, pulled by Sarah on a tricycle, while throwing Mardi Gras-esque beads to stuffed animals along the lane. The "carnival" is in fact a celebration of the end of the school day.
It was first and only seen in "Mission Ed-Possible," so it is unknown if they do this after every school day; the episode presumably took place at the end of a semester, as the kids received their report cards that day. In the episode, the celebration was interrupted by Edd, who was racing away from his friends. This led Sarah to chase Ed and Eddy after Jimmy alerted her to "rabble rousers", which required her to abandon the float and the parade.
It is unknown if, after the chase, she returned to finish the carnival or if Sarah and Jimmy simply gave up on it and went to do something else.