Teachers can offer some questionable trivia (an old teacher of mine once claimed that bipolar disorder was an outdated term for schizophrenia, to give one example), but when it comes to the actual content, they're usually quite secure - or as secure as they can be, given all the changes to the system. Still, if you're wary about the teacher's grasp of the content, then you could use that as extra motivation to read ahead before lessons (which is a very useful habit to get into anyway), so that you can spot any mistakes the teacher might make. I found that CGP revision guides were a good supplement to the textbook if you can get hold of any of them.