Hi Mark,
Is the school going to take photographs? If so then the piggyback may work, if not it will be harder to use for visual for 2 students than a side by side system.I recently had to solve the same problem with a vixen side by side plate and countersinking the screws into the vixen dovetail bar with a 90 degree countersink drill bit allowed the dovetail to sit flush in the saddle and gave better grip.
I am also on the Sunshine Coast and happy to show you what i mean if it will help. I also have the right drill bit.
John