答案：B.
翻译：学校自己也承认,不是所有的孩子都会在他们为之而正在接受培训/训练的工作/职业中成功.
1.定语从句如果使用关系代词,必须记住关系代词等同于先行词,并在定语从句中充当一定的成分.
2.此题中,which = the job,做介词for的宾语,代入定语从句中为：
for which they are being trained
= they are being trained for which
= they are being trained for the job
句意为：他们正在为了这份职业而被培训/训练
3.如果使用in which,则定语从句为：
in which they are being trained
= they are being trained in which
= they are being trained in the job
句意为：他们正在工作中被培训/训练
4.两相比较,肯定是for which更符合句意.