How would you describe silence outside the school gates, before the school is about to finish?

I'd call it a pregnant silence.

why pregnant?

is it because the school will soon have all children coming out?

Yep! :-)

http://www.oxfordlearnersdictionaries.com/us/definition/american_english/pregnant

To describe the silence outside the school gates before the school is about to finish, you can follow these steps:

1. Observe the surroundings: Look at the area surrounding the school gates. Take note of any physical attributes that contribute to the silence, such as absence of students, reduced traffic, or calm weather.

2. Note the time: Consider the time of day when the silence occurs. Typically, this would be towards the end of the school day when classes are about to finish.

3. Consider the anticipation: Think about the emotions and behaviors associated with the imminent end of the school day. Students may be eagerly awaiting the final bell, leading to a focused and subdued atmosphere outside the school gates.

4. Incorporate sensory details: Pay attention to the sounds, or lack thereof, that contribute to the silence. This may include the absence of laughter, voices, or the typical bustling noise that accompanies students entering or leaving school.

By combining these observations, you can describe the silence outside the school gates before the school is about to finish in a way that reflects the atmosphere and captures the essence of this specific time and place.