2009 Megane clutch slipping?

This is my fourth Megane, no problems with first three, which I taught 30-40 driving lessons aweek in. This one I teach around 5 lessons a week in, theres 21k miles on it and now the clutch is slipping. Renault say its not under warranty as its not a fault and theyre looking me to pay £1K for a new clutch. They say 'someone' must be riding the clutch. If this was the case, my pupils wouldnt pass their test and all of my cars wouldve needed new clutches. Help please!!!