Skippy:
I think you need a lot more info than that.
1) How has it been maintained? Sure, training aircraft have to be L2 maintained, but there are L2s and there are L2s, if you know what I mean. The appearance of the aircraft, especially under the cowlings will tell you something about this.
2) Was it hangared or left outside? What shape is the paintwork and the glareshield top in? Is there any evidence of corrosion? How do the windows, especially the windscreen look? Scratches? Discoloration? Cracks etc?
3) What is the general condition of the interior?
4) How does the engine run? Does it use oil? What condition is the prop in WRT scratches, chips etc. Has the prop been repaired for stone chips? Has the aircraft ever had a prop strike?
I could go on pretty much all day with this, but I think you get the drift by now.
If you know a good L2 who hasn't been involved with the maintenance of the aircraft, get them to do a pre-purchase inspection and report. Then if it's a common aircraft like a Jab or a Tecnam (which most school aircraft are) have a look online to get a feel for the value of the aircraft. Then armed with the report, start negotiating with the seller.
The short answer is: there's no formula for discounting a used aircraft.
The right price is what the seller and the buyer agree on.