This article adds to computer-assisted language learning by extending a study on an essay critiquing system (ECS) feedback to secondary school language learners' writing. The study compared two groups of participants' performance, namely the treatment group which received both the system feedback and teacher feedback (i.e., blended learning mode), and the control group which received teacher feedback only. The study was conducted in a secondary school in Hong Kong in the form of an extra-curricular activity after school. Fifty-three students from the ages of 16 to 17 with different argumentative writing experience participated in a series of five writing workshops once a month voluntarily. The participants were assigned to a treatment and a control group based on the stratification approach. Their scores were compared and an interview with the treatment group was conducted. Analysing the total and content and organisation scores of the two groups, it was found that both groups demonstrated statistical significant gains. Nevertheless, the treatment group's gain appears to be more meaningful than the control group's in view of its group composition. The blended learning mode contributes to the encouraging results as revealed through the treatment group's scores, high rating and positive comments on both system and teacher feedback in the survey and interviews. The analysis also implies the need to improve the system feedback on paragraph coherence, workshop design and grading criteria.[Copyright of Computer Assisted Language Learning is the property of Routledge. Full article may be available at the publisher's website: http://dx.doi.org/10.1080/09588221.2011.630672]