Q:

What does "call BS" mean in the sentence "We call BS"?

What does BS mean in the sentence "We call BS"?
I saw an article about protests in the US that said the following (see here the video):

Florida student Emma Gonzalez to lawmakers and gun advocates: 'We call BS'

I've checked Cambridge dictionary and I found that BS is an abbreviation of "bullshit". Then, now it's absolutely not understood to me what "we call bullshit" means.

A:

The expression is used to call out (= to draw attention of others upon) a lie or a negligent or deceitful mistake.
As you have found already, BS stands for bullshit, a profanity that basically means "nonsense".
The verb "to call" can mean "to cry out", and it is often used when someone says something short in order to stop everybody else from going further. For example, a refree can call "foul" when playing some sport.
Therefore, "I call BS" means "You are lying/wrong, and I'm telling everyone".

A:

To call is to declare a decision or judgment, especially in a game or contest, but in any context where the participants are expected to abide by certain rules. 
For example

The referee called the ball out.  (sport)
I call heads.  (a coin toss)
I call foul.  (the speaker believes his or her adversary has broken a rule or has acted in an unsportsmanlike manner)

So, to "call B.S.", is to declare such a judgment about another person's behavior.
In other words, I declare that you are speaking BS (bullshit, tripe, nonsense).  The rules of proper civil debate require participants to take cogent positions that don't distort the facts.

A:

You say I/we/etc. call B.S. when someone is being insincere, untruthful, or when something is false, misleading, or some similar circumstance. It generally means you don't believe someone. You can similarly just say "B.S." or "bullshit". For example

A: I ate three cheeseburgers for lunch today.
  B: I call B.S. Two, maybe. But not three.
A: I pulled an all-nighter studying for this test.
  B: Bullshit! You can barely stay awake past 10.

Regarding the OP, Gonzalez doesn't believe the politicians when they talk about gun violence. For example, she says in this video (CNN)

They say that tougher gun laws do not decrease gun violence. We call B.S.!

