Navigate to Courses Screen.
Select the course with the error.
Select the Skill Set button and double click on the skill line that appears. Delete the skill.
Delete the subject, as well, to avoid further errors.
The reason that these errors are appearing is that in the Courses listed for these students such as ALG1 there is a skill set that is tied to the course. When you select the skill, the subject is listed with the skill being blank. The skill is tied to the skill ID that is showing for the error on the dashboard.