"Of Whom" or just "whom"

Trying to settle a bet. In the following sentence, "of whom" probably would have been a better choice, but the writer argues that even if it isn't the best way it is, technically, acceptable:

"The school had a record 355 students this year, many whom received free lunches."

If doing it again, would have said "many of whom" but that isn't an option. Just wondering if it could be correct anyway, just a bit awkward.

Thanks in advance,

Jeff