I am a parent that has multiple students at multiple schools that use Blackbaud (CompassNet is Harding Academy's branding of Blackbaud's SIS 'Student Information System') and I am having trouble logging into these separate accounts for different schools.
Use a different web browser for each school. For example, if your student 1 goes to Harding Academy - use Chrome to login to hardingacademy.myschool.com and use Safari (Mac) or Edge (Windows) to login to SecondSchoolName at secondschoolname.myschool.com. (SecondSchoolName is a fictitious name) for student 2.
You may have to clear your cache and cookies in your current browsers to get this to work.