Thank you for this important question, Raymond Chen.
Unfortunately, it is not possible to prove how many hours were needed to make a given set of captions or to provide translated subtitles. It depends on too many variables: audio quality, complexity, captioner/translator's experience.
You should discuss with your school how to evaluate your captioning/translating work. Could you and them agree on a formula like "Captioning/translating time = video length x 3 (or another rough constant)"?
Best,
Claude
Raymond Chen
How do I get proof of hours done doing captions and translating for videos? I need proof of at least 50 volunteer hours in order to graduate my high school.