Any English equivalent for the Persian idiom "to play dead like a mouse"?

I'm looking for an idiom or expression for describing someone who fools or manipulates other people by pretending to be poor/ weak or incapacitated/ ill (=unwell)/ inoffensive / innocent as a tactic to reach their aims or goals, because they know that in such conditions, it is more likely that people react more kindly and thus help them to reach what they intended to gain or receive (for example more money, more salary, more help, more support or attention, etc.)

For example:

1- Two bully students have been fighting in the playground, when they are called to the pricinpal's office one of them keeps silent, and pretends to be inoffensive and just being beaten and not have beaten the other student, in order that the principal not scold him and only the other student would be blamed and punished.

2- Beggars use this act/ tactic to make people feel pity for them and help them more.

3- Sometimes kids act like this in order not being punished by their parents or teachers.

In Persian we use this idiom " to play dead like a rat/ mouse". (As you know, the mice use this trick as a defense mechanism when facing a threatening situation, and scientifically is called "tonic immobility".)

Like in:

"Come on! Don't trust him, and don't be fooled by his innocent look either. He is just playing dead like a rat!

"Don't be fooled by her, she is just pretending to have low back pain, she is healthier than anyone in the office! Let her do her own job on her own. She is just playing dead like a rat!"

Is there any idiom or expression that can be used for describing these kind of sly and cunning people who reach their aims or goals by pretending to be weak, poor, inoffensive, innocent,...?

I have found "to play the innocent" but it means "to pretend not knowing something", so I don't think it could be used in those situations too.

Answer

I'm not exactly sure that the idiom of playing dead like a rat translates to being cunning, but playing dead/pretending to be asleep to avoid injury or confrontation has an equivalent in playing possum.

Your request sounds more like "playing [the] innocent":

"Forgive me, I do not understand," said Cicero, playing the innocent. - NYT
John is playing innocent, and he knows more than he is telling us.

Even more common is playing dumb:

To pretend that one has no or little knowledge (of something); to act ignorant or uninformed (about something).

Another is "dumb like a fox". Foxes are clever foes. When one is pretending not to be clever, this can be used. Also used, in a similar situation, is "crazy like a fox." But these imply hidden cleverness as a disguise, and @Edwin Ashworth's answer is a more common idiom for when really disguising yourself when you have predatory intent.