Student drivers are required to keep a log of their supervised driving hours. However, not all students are diligent in their record-keeping.
The transition that should be used to combine these sentences is the following one: however. This adverb is used to introduce a statement that contrasts with or appears to contradict something that has been previously said. In this case, you are saying that student drivers should keep a written record of their supervised driving hours yet you are also stating that not all of them do it thoroughly and well.
