Hi,
So yes (and make sure I understand) (P.S. I would use SharePoint myself versus the Excel, long list of why)
Let's say you have Table 1 and it has the Contact Information for a Parent (aka the email address).
Then let's say have 20 other columns. so a total of 21 columns in Excel.
In the other 20, you would put in columns for the 15 questions and have 5 left over for student into
Essentially you would have Column 2 (named Answer 1 Details) or whatever..
As you say, they are all blank initially.
Now when someone submits, you will do a List Rows in a Table, and the fitler is
EmailColumnName eq 'The_Dynamic_FormProperty_Submitter', which has the email address
and then you would have a condition action
the left side, would be length(Dynamic body/value from your List Rows)
middle is equals
right side 1
This checks if you got 1 row back
in the Yes side of the condition
You would then use the data from the Forms to update the excel columns.
Bingo and done :-)