Public Schools don’t have the authority to just suspend or expel a student at their discretion, remember they are government entities and must comply with constitutional safeguards like due process (notice and an opportunity to be heard). That’s why they have a discipline matrix in student handbooks. These typically have graduated punishments prior to suspension. I don’t know about Michigan, but the state where I live has a statutory public policy not to expel except in egregious circumstances. In this case, it appears that the student had general, non-specific threats and signs of a mental health problem/crisis. You’re not going to sustain a suspension for that as the threats will be considered minor and mental health isn’t a disciplinary issue.
Hence the reason they should have assigned the kid to online classes and sent him packing.