I think you might confused the term PPA here. You shouldn't be allowed to use ANY of your PPA time to cover staff. Each member of staff should have a certain number of PPA periods a week, which are protected: as far as I know they're not allowed to use you for cover in those. I think it's supposed to be half a day a week or something like that - at my school everyone gets 2 hours a week PPA. In addition to that you probably also get some more non contact time that is not actually PPA, so you can be used for cover in those. It's very unlikely that you have 6 PPAs a week - you may have 6 free lessons 2 or 3 of which are actually PPA. It sounds like your timetable is not compltely full so, annoying as it is, they are allowed to use you for cover lessons. As an NQT your teaching timetable should be reduced by 10% compared to a full time teacher with no extra responsibilities. However this doesn't necessarily mean you get this as extra PPAs. It sounds like you need to find out which of your free periods are PPAs where you can't be used for cover. Then if they try to make you cover during your PPA you can just say, "there must be a mistake - this is my PPA"