Sugestão de correção:
When I was in High School, I went to school by bus. However, the bus sometimes was late and I arrived late at school. The problem is that at the school entrance door, there was a supervisor who stopped late students from getting into the school until the first class finished. One day I had to do a presentation to the teacher and that guy decided definitely to prevent late students from getting in. He sent every late student back home. I did not have a choice. I had to wait for him to be distracted and I sneaked inside school.