'Speak out' is unnatural and wrong in the OP's sentence.I have heard "speak out" and "speak up" with the same meaning. I don't see how "speak out" is wrong.
'What can we do to help children who are too shy to speak out in class?' Would seem to be OK in AmE.Wrong? I went to a seminar for parents with kids who hold back and hesitate to speak out in class.
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