Peer feedback is proved to be effective in helping enhance students’ writing quality, but few studies were employed to train high school students to be better peer reviewers. The current study involved 64 grade-11 students in a province in Vietnam to see whether trained peer feedback could be effective for high school students. The experimental group was trained to conduct peer feedback, while the control group did it naturally. The semester lasted for 16 weeks. Data collection was from the pre-test, post-tests, and semi-structured interviews. The results revealed that the most common errors that the students committed were grammar (verbs, articles, repositions), followed by vocabulary (word order, word choice, word form), and mechanics (capitalization, spelling, punctuation). In addition, the students in the experimental group who received peer feedback training could significantly reduce the written errors in the post-test. The students obtained positive attitudes towards peer feedback activities in the writing classroom.