Final answer:
The figure of speech used in the student's sentence is irony, which plays with the contrast between expectations of adults and the narrator's realization that adults are not as they seem.
Step-by-step explanation:
The figure of speech used in the sentence "Was it the time I realized that adults were not all they seemed to be." is irony. Irony often involves a contrast between expectations and reality, typically in a way that is somewhat humorous or thought-provoking. In this case, the expectation might be that adults are knowledgeable, capable, and have everything figured out, but the realization that they are 'not all they seemed to be' subverts this expectation, pointing to an ironic understanding of adulthood.