Ah, RamAirTurbine, if only the interiors were cheap to maintain! Some of those little bits of plastic cost telephone numbers, if you can get them at all. (anyone got the bit for the Arrow, by where the MAP guage and gear lights are?! AAAARGH can't find one anywhere!)
Seriously though, it is important to keep the aircraft looking good, lots of TLC shows that the school takes pride in them, and they will probably have good quality in other areas too. A clean aeroplane shows a dedication to customer service, which many schools sadly lack, however good their instructors are... it doesn't help if the bookings are in turmoil, planes come back late and the aircraft run out of hours on Friday afternoons...
In defence of old aeroplanes, we have some on line in their thirties, and if they are well looked after they can go on for ever. In fact some of the "old ladies" are nicer to fly than the shiny new ones. Bear in mind that there are probably few original parts inside if they are kept up to the airworthiness limitations.