It should say something in your contract. If the contract (or letter extending your probation period) does say that you automatically become "permanent" on passing your exams, you can see your employer's point. However, if the contract or letter says that you will be informed of your "permanent" status, then if you resigned prior to being told that, my view is that the 1-week notice is sufficient.
Obvs bear in mind the potential damage caused by irritating a former employer - if it was me (assuming there was nothing in the contract clearing this up), I'd try to compromise on it.