To get approval to become an IB school, the principal and teachers have to attend workshops and get some professional development. But IB does not specify that teachers have to meet any standard other than being certified by the state. Compare this to the College Board's recommendations for AP teachers: a major in their subject, at least three years of experience, and National Board certification (or, at least, have the characteristics that such certification identifies).
Independent appraisals of the effectiveness of IB programs are hard to come by. IB itself recently produced a study that showed that, after controlling for race/ethnicity, income and performance in high school, IB high school grads did better than their peers in the UC system. It's methodologically difficult to control for all the factors at play in college success, so this claim should be taken with a grain of salt. But whatever the actual results are, it's probably safe to conclude that if IB ever took teacher effectiveness more seriously, it would have even more successes to celebrate.