Be sincere when you ask for leave, and promise the teacher that you won't delay your study. It is best to pretend that you don't want to ask for leave, but you have to ask for leave. If the teacher is in a good mood when taking time off, it is better to take time off. Another thing is to see how many times you usually ask for leave. If you have too many times, it will be a bit difficult to ask for leave. If there are not many times, you'd better ask for leave.