Below are a few steps you can take in order to prevent students from unenrolling themselves from individual assignments in Microsoft Classroom:

1. Navigate to the individual assignment in Microsoft Classroom for which you would like to prevent students from unenrolling themselves.
2. Once in the assignment, select the Coursework tab from the top.
3. In the Actions column for the assignment, select the three vertical dots.
4. From that list of options, select Require student to ask for help before unenrolling option.
5. Once clicked, a verification box will appear for confirmation of the action.
6. Select Yes to confirm that you want to enable the feature.
7. Once the feature is enabled, students will have to ask for help before they can unenroll from the assignment.