If you compare the running costs of a 2 seater to a 4 seater I think you'll find there's not a lot in it.
The airframe and engine maintenance costs will be very close, the capital cost will vary and so will the fuel burn.
The extra utilisation created for the 4 seaters by only having one type probably helps out weigh any extra costs of the 4 seater.
If I were a flying school owner I wouldn't bother operating a 2 seater. 4 seaters will cost very little more to run and provide so much more flexibility and versatility.