Is it acceptable to start an emphatic sentence with "It is he who…"?
Solution 1:
The sentence structure is
Subject (it) - Copulative Verb (is) - Complement (he).
The complement should be in the same case as the subject, so "he" is correct.
It's the "who" that's problematic. The structure of the relative clause is
Subject (students) - Active Verb (chose) - Direct Object (whom)
In formal writing, the pronoun should be in the objective case because it stands as an object. In less formal writing, and certainly in casual conversation, people substitute the nominative "who."