Well as with any externally moderated course, the IB has really specific standards and criteria for what they want, which aren't always clear from the marking scheme - or the teaching agenda, the contents of which is not always reflected in exams. Basically you want teachers who are experienced enough to know WHAT constitutes a good IB grade, WHAT it is that the IB want and also what they
don't want
Being a guinea pig student in a school is a dangerous thing because the teachers know nothing about the course either. So often the first few years don't do as well, purely because they received poor guidance. Teachers are meant to go on courses about how to teach the IB, but not all of them do and for some of them, I honestly don't think they care.
Poor guidance you can kind-of compensate for as a student (and IBS is a great resource to find out from other people who have more experience WHAT it is that you should be doing!) but at the end of the day, the teachers mark your work. They could give you 7s all year and then when your work goes off to the IB moderators, it's all moderated down to a 4/5 because your teacher didn't know how to assess it properly. I had this with some of my subjects - when you're being predicted a 7, you don't really think it'll be moderated down to a 4!

You just trust your teacher implicitly... but if your teachers have no clue, you're kinda screwed. You have no idea how well/badly you're actually performing so you can't improve and you think that things are correct when actually you're going about it all wrong.
The majority of teachers get the hang of what works/doesn't and see how their own marking has been remarked by IB examiners etc. - but obviously this takes at least a year of intake before they can start learning how better to teach and assess you!