This paper describes an exploratory trial conducted with 14-15 year-old students in English state schools who were studying business start-ups as part of a course in "Business Studies". The intervention uses a "Story Grammar" strategy to improve students' reasoning by increasing the frequency and complexity of their use of "connectives" such as "when", "if" and "because". The analysis reports positive effects of the intervention on students' understanding as judged by the use of a standard examination style mark scheme, and the number and complexity of connectives used by students in their extended writing. By reporting effects on immediate target variables we are able to examine the causation with precision. Effects are reported through standard t-tests and effect size. We also discuss the design of the experiment and comment on its practicability as a model for investigating effects of classroom interventions.