Don't force a good habit, help it grow instead. For example, ask them to fix bugs in a prepared project with a number of subtle errors without unit tests and c

Don't force a good habit, help it grow instead. For example, ask them to fix bugs in a prepared project with a number of subtle errors without unit tests and comments, then another project with good tests and comments. After that, when you give them a group term project, not only will they be graded by you according to the code standards you set, but their peers will demand and expect that they write the code well.