In = Period of Time, Year, Season, Month, Part of Day
I went to school in the past.
I went to school in the summer.
I went to school in 1974.
I went to school in March.
I went to school in the morning.

On = Specific Day, Specific Day + Part of Day

I went to school on March 10th.
I went to school on Tuesday.
I went to school on Tuesday afternoon.
I went to school on the morning of March 10th.

At = Specific Time, Specific Time + Day

I went to school at 10:00.
I went to school at noon.
I went to school at noon yesterday.
I went to school at two o'clock.
the animated image to identify expressions of time
Animation by Koon Wei Ho