Fifty of the more than 300 children snatched by gunmen from a Catholic school in Nigeria have escaped their captors, a Christian group said on Sunday.Gunmen on Friday raided St Mary's co-education school in Niger state, taking 303 children and 12 teachers in one of the largest mass kidnappings in Nigeria.The abduction came days after gunmen stormed a secondary school in neighbouring Kebbi state, abducting 25 girls.