David is insisting that Laurie’s mother is wrong about Mr. Ross. Is David right or is Mr. Ross slowly brainwashing them as Laurie’s mother suggested

Mr. Ross doesn't mean to "brainwash" the students. His social experiment was meant to prove a point. Laurie's mother is correct that an unintended consequence of Mr. Ross's idea becomes a form of brainwashing or hysteria. Mr. Ross eventually realizes this and ends his Wave experiment.