Free Download Per Passcode?

I am getting ready to set up my fall school and sports galleries. Everything is a pre-pay base, with everyone in the school or club receiving the same base package. (Invoiced to the school, so no need to set up pre-pay in the gallery.)

Most of the schools will have a digital image as the base purchase and I'm wondering if there is a simple way to set up a free download per passcode? This would allow me to photograph 3 poses, but only provide one of them as a free download to the family.

Something that might work is have a Free Download product and there is a setting to limit how many free downloads per gallery. 

Assign that download product to the price list for the gallery  and who visits will get one free download. 

I would recommend setting the free download to where the customer has to be logged into an account for this.

See: Free Downloads

In this case would the download limit "per gallery" convert to "per passcode"?
The entire school will be in the same gallery, with each student being a different passcode.

It would be per gallery, not per passcode. Not an option for per passcode. 

If you require an account for the free download, it will be per account per gallery. If not requiring an account , per IP address per gallery.

Okay, I'll have to find another option then as that won't work with siblings in the same school.
Thank you!