While taking your driving lessons you must make sure your driving instructors vehicle you are taking driving lessons in is taxed. This road tax should be in every vehicle on the road unless it's exempt for certain reasons.
Road tax in the past was calculated by your engine size, but times have changed and now the concerns for the environment are escalating, road tax is now mainly calculated by the amount of CO2 emission levels your car pumps out.
Some driving schools try to use more economical cars to save the environment, as well as to save you money when taking driving lessons. This also lowers the CO2 emissions and reduces the amount of road tax you will have to pay.
You can see your cars CO2 emissions by just simply referring to the logbook and looking for CO2 emissions. Brackets are different for both new and used cars, however similarities between them are: