12 noon is neither a.m. nor p.m.
To avoid confusion, the correct designation for 12 o'clock is 12 noon or 12 midnight. Alternatively, the 24-hour clock system may be used.
The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line). At 12 noon, the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither 'ante-' nor 'post-'.