In schools at the time of assembly a good thought is shared with the students. Now my question how it should be announced... "THOUGHT FOR THE DAY" or "THOUGHT OF THE DAY".
The answer depends on your intended meaning. "Thought of the day" is one of a daily series. "Thought for the day" is a thought which is specifically for today. For example, if there is something that happens every Tuesday, your "thought for the day" might be specific to that day.
In your specific example (daily school assembly), I would use "of". Either word, however, would be appropriate here.