They may start as a group of high-school students？
may start 谓语动词，may情态动词表可能性，后接动词原型start，as介词，此处表示＂作为＂，后接名词，名词通常表示身份等。for whom 引导非限制性定于从句，前有逗号与主句隔开，定于从句的先行词是students，人，所以从句引导词优先选用who或whom，因为此处引导词作介词for的宾语，所以可以用whom这种宾格形式。
这句话拆分后就是：They may start as group of ...students. For those students, practicing their music .....to fame.
