We review the impact of an online course on gravitational-wave astrophysics aimed at high school students. We launched the video course in 2021 on the web page Webuni in Hungarian. Since then, there have been more than 300 participants attending the course. Our case study aimed to determine how well such a course can give an overall picture about the topic and to measure how the attitude of Year 9 and 10 students to physics change with their participation in the course. The data were collected from 22 students in the academic year 2021/2022 with surveys measuring the changes in knowledge and attitude towards physics. The students showed significant improvement in both lexical and conceptual knowledge by a factor of 2.2 in test-scores and found the course easy to follow (rated 4.2 out of 5). We observed significant change in attitude only towards ‘following of gravitational-wave related news and discoveries’, from 1.8 to 2.8 on a scale of 1–6. The participants considered that they enjoyed (4.3 out of 5), found useful (4.4 out of 5) and would recommend (4.4 out of 5) the course to others. Most of the students (92%) in the sample tested were from Hungary’s capital city and two thirds were girls, therefore our results are not representative of the Hungarian student population.