Notice during the “tack scene” in the principal’s office, Helen is not holding Jack Jack:
Then, in the next scene, while Dash is begging to play sports, Jack Jack is quietly sitting in the back seat:
As far as we know, the Parrs had no friends (presumably b/c they were superheroes in hiding) and did not use a babysitter. (Kari did not come along till Violet hired her to watch Jack Jack so she and Dash could hitch a ride on the Jet.)
Helen is a very loving, protective mother; so where was Jack Jack while she was in the principal’s office?