Information flow type systems provide an elegant means to enforce confidentiality of programs. Using the proof assistant Isabelle/HOL, we have specified an information flow type system for a concurrent language featuring primitives for scheduling, and shown that typable programs are non-interfering for a possibilistic notion of non-interference. The development, which constitutes to our best knowledge the first machine-checked account of non-interference for a concurrent language, takes advantage of the proof assistant facilities to structure the proofs about different views of the programming language and to identify the relationships among them and the type system. Our language and type system generalize previous work of Boudol and Castellani, Theoretical Computer Science 281 (2002), 109–130, in particular by including arrays and lifting several convenient but unnecessary conditions in the syntax and type system of the work of Boudol and Castellani. We illustrate the generality of our language and the usefulness of our type system with a medium size example.