Generally, it won't affect you, because the school will usually take out your file before the college entrance examination. If there is a demerit, it will refer to your usual performance and will generally cancel it for you. It's okay. Don't worry if you behave well at ordinary times. If you don't behave well, be a good boy in senior three and get on well with your teacher. Then ask the teacher to help you, or give you some sweetness, and you will be fired.