As always, a 28ga semi is a very good way to go. Unfortunately if you don't have the gun, or access to 28ga ammo this can be a problem.
Recoil is a real SOB when teaching new shooters. You tell them to put their face down on the gun so they can see down the rib and get a good idea of where their shot is going. After they pull the trigger, the gun comes up and smacks them on the cheek, and they never will put their face on the gun again..... If they get lucky and have a good fitting gun that doesn't hit them in the face, then their shoulder is going to get pounded.
What were you using for 12ga ammo? Regular AA target loads give my head a solid shake after 100 targets. Regular Challenger will make my nose bleed (ok, maybe not, but the stuff is HOT). There are low recoil and low payload options available that may help. When I was still shooting 12ga, I had a good 1oz reload that was light in recoil.
If the 12ga is your only option, you could try some different ammo selections.
I own a lightweight 20ga Wingmaster (LOVE hunting with it) and the recoil on it with regular loads I have found to be like my 12ga reloads. There is still significant recoil. If you could find a 20ga Semi (which may be easier to find than a 28ga) that would help.
Brad.