By Fateh Al-Rahman Al-Hamdani
BBC News Arabic

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When I meet Ahmed, he is shackled in a room all alone. There are marks on his body from the beatings he has been given. He doesn't know how old he is, but he's probably about 10.

The school I find him in is one of 23 Islamic educational institutions in Sudan, known as khalwas, that I filmed in undercover over a two-year period, starting in early 2018.

I witnessed and filmed many children, some just five years old, being severely beaten, routinely shackled, and imprisoned without food and water by the sheikhs, or religious men, in charge of the schools. Some of the children who did not appear in our documentary told me they had been raped or experienced other forms of sexual abuse.