Do I need to report students absent via SMS?

No. UKVI have said that “You do not need to report student or employee absences related to coronavirus.” They confirm that this includes absences related to illness, the need to self isolate or inability to travel due to restrictions.

The Home Office have confirmed that they will not take enforcement action against sponsors who continue to sponsor students despite absences due to coronavirus. They do confirm that you must keep a record of any absences that you authorise, along with any appropriate evidence that the absence was due to the current outbreak.