This has a diesel particulate filter and almost inevitably what will be happening as a result of your low speed running as a driving school car is the DPF has to actively regenerate and the engine will not run normally while this happens. It's virtually unnoticeable except from the instant fuel economy readout on a motorway. But in stop/start running you will feel it. Open a door or window and you will probably be able to smell or feel the additional heat generated. You can't get away from this problem with a diesel engined car first registered on or after 1-1-2011.