Translation:Tell the teacher that I am going to be away tomorrow.
You might remember the structure that means someone is absent or not found at some place:
- Его нет ~ He is away (one of the interpretations).
- Его нет дома ~ He is not at home.
- Учителя пока нет. = The teacher is not here yet/There is no teacher yet.
It works the same way in the past and in the future (well, unless you take "being" somewhere more actively, as "going" to that place). If the person did not or will not show up somewhere, use the Genitive with не было or не будет. You may state the place explicitly or leave it to context:
- Его не было дома. = He wasn't at home (e.g. I went there and did not find him)
- Меня завтра не будет дома. = I will not be at home tomorrow.
- Его нигде не было. = He was nowhere to be found.
- Послезавтра меня не будет. = I will not come the day after tomorrow (to the place understood from context)