High percentages of newly qualified teachers (NQTs) drop out during their first 5 years in the classroom. Often, formal support systems are put in place to overcome 'practice shock'. However, in this research, it was hypothesised that it is not the formal support structure put in place that determines whether starting teachers feel satisfied in their job and show perseverance but rather the amount of knowledge exchange that takes place. This was confirmed by the results of a first quantitative study. Then, a follow-up qualitative study showed that having the principal in the role of a mentor is often experienced as a mechanism of control or evaluation. Starting teachers prefer to choose their own mentor. They prefer their mentor not to be a superior but a close colleague whom they trust, who is teaching the same course in the same year. Our results have especially implications for onboarding of novice teachers. Since social informal learning (e.g. through the exchange of feedback with colleagues) benefits newly qualified teachers, it is important to create a safe and warm learning climate in which knowledge exchange can flourish. Also, NQTs should be given the opportunity to choose their mentor.