Restricting capabilities of admin role in database schema
Generally, you cannot prevent administrators from making any change they want to the database.  Many administrator actions require privileges that give them a lot of control over the database.  For instance, if they can restore from backups (something that administrators often need to do), they can overwrite everything in the database.

Designing systems that will remain secure even when the system administrator is not trusted is very challenging, and for a learning management system, is probably more expensive than is worth it.

Therefore, there's probably no point in trying to restrict admin.  Don't forbid what you can't prevent.