It is a good idea to encourage teams to take as much responsibility for their preparation for debates as possible. The more they research and work independently, the more familiar they will become with the material - this is a great help when they are actually debating, as they will be better able to manipulate that material during the debate.
Some coaches will effectively write the pupils’ speeches. This is bad for a number of reasons and should be discouraged. It goes against the spirit of most competitions and stunts the young people’s development and it will usually be spotted (and penalised) by adjudicators.
General pointers are very useful, but the pupils really do gain much more benefit when they are responsible for their own speeches and it helps them develop in the long term.
However, that isn’t to say that a teacher cannot help a team in preparing for debates:
If possible, it may help to add extra sessions in to help the team.