Please (do ~) / Could you (do ~) / Could I ask you to (do ~) / Would it be possible for you to (do ~)
The above phrases are appropriate for use when you have a specific request to make of a teacher. If, on the other hand, you do not have a specific request (for example, when you explain your child's situation and ask the teacher to take appropriate action depending on what happens at school), it is better to use requests like 「すみませんが、よろしくお願いします」(sumimasen ga yoroshiku onegai shimasu, or "I'm sorry, but thank you") or 「お手数をおかけしますが、よろしくお願いします」(otesu o okake shimasu ga yoroshiku onegai shimasu, or "I'm sorry to trouble you, but thank you"), which are more open-ended ways to make requests or ask for cooperation.