How Can I Exclude Vacation Weeks From A Week Number Count?

I am creating a Google Sheet for teachers that automatically fills in the week number beginning with the first week of school in early Sept. and continuing to the last week of school in mid June. I am currently using WEEKNUM, which is working well, but I am anticipating that this will become inaccurate after vacation weeks. I want to exclude these vacation weeks from the count, but WEEKNUM will include them.