To what extent school improvement processes can be initiated and sustained from the outside has been a relevant question for policy-makers seeking to increase quality in education. Since 2008, the Chilean Government is strongly promoting the use of external technical support (ETS) services to support school improvement processes, as part of the Student Priority Voucher programme, a compensatory policy that provides significant additional resource to schools serving the poorest 40% of students. Participating schools and ETS providers negotiate services and prices in a market-like dynamic. In this paper, we describe the Chilean ETS policy, discuss the ETS policy implementation, review the empirical evidence about the effects of ETS services in schools, and critically analyse the ETS policy as an instrument for school improvement. We argue that experts who are knowledgeable in the field of education have the potential to effectively support schools in need of improvement. However, brokers and external consultants providing school support in Chile within a market-oriented framework typically lack the characteristics associated with positive effects; this raises the critical issue of developing professional capacities within schools to produce genuine and sustainable improvement processes.