Why do we say 'most of whom' instead of 'most of who?'

I was editing a paper today, and I came across this and although I've always always seen 'most of whom' in any sort of writing before, it just doesn't seem right.

"...the offices will be used for assistants, most of whom will be students.'

If we're saying that 'whom' is the object here, where is the subject for 'will be?'

Is 'most' the subject? or is 'the assitants?' You would never say '...the offices will be used for assistants whom will be students,' it's obvious that the 'who' replaces 'the assistants' as the subject, and therefore we use 'who.'

No matter how I look at it, it seems that 'most of' should just be an adverb to the phrase, and 'who' should become the subject for the verb 'will be.'

My two cents? Most professional writers learn informally the difference between 'who' and 'whom' and are very quick to throw 'whom' when a preposition is involved.

Am I missing something here?
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Comments  (Page 2) 
Yes, most is the subject.