PA-28 flight Instructor reported that while their student was performing a solo landing; they had a bird strike on final approach. During landing; the student lost aircraft control resulting in a runway excursion.

Narrative

In the morning; after discussing with my student and combing through various weather reports to make an appropriate go/no-go decision; I made the determination to send my student for a solo; takeoffs and landings; to a full stop. This was to be her second solo in a 141 program. According to the ASOS and METAR; weather conditions for the morning were appropriate to a pattern solo - (according to the ASOS - called prior to walk around) winds variable at 5 knots; 10 sm of visibility; pressure 30.09; density altitude 1;675 feet. We conducted an analysis of the aircraft's performance data; located in the 'performance and limitations' section of the POH to account for the day's aircraft performance. Following that; we filled up the tanks from the local FBO; and proceeded with the aircraft walk around. There were not any items damaged on the plane prior to takeoff and the aircraft was not due for any maintenance. After that; my student entered the aircraft; and departed for her second solo.After her runup was complete at the runup area; I watched her taxi for the first circuit in the pattern of the day. She was cleared for takeoff; and proceeded to takeoff and make left closed traffic. I was monitoring her progress on foreflight; and listening to her radio calls on the tower frequency. After being cleared to land; she appeared stabilized throughout her descent both on flightaware (ADS-B); and visually; and made her first of 4 landings. She adhered to my instructions; decelerating to a full stop before taxiing back to the runway for full length departures. Her second takeoff was also very stable; and once again commenced left closed traffic. On the second circuit in the pattern; she was extended downwind before being cleared for another full stop landing. While she was outside of my direct sightline at this time; in reviewing the ADS-B data; she appears to have initiated a stabilized descent at airspeeds consistent with her training. About 50 feet over the runway and prior to touchdown; my student states she 'hit something and there was a loud thud on the right side of the plane.' Upon debrief; she states that she believes it was a bird or possible drone strike. In the moments following the strike; she says was unfocused and continued with her landing instead of proceeding with a go-around procedure. While attempting to regain control; she suffered from a runway excursion; and hit a Runway Distance Remaining sign while departing the runway. The tower sent assistance; and she was able to get out of the grass and taxi back to the ramp under her own power. Upon post flight walk around; there was substantial damage to the aircraft's left wing; and what appeared to be an impact point of the bird strike on the right side of the cowling. According to her training; she has logged countless takeoffs; landings; and go-arounds - consistently demonstrating premier procedural knowledge of traffic pattern operations; and radio communications. During her pre-landing brief; she states to expect the go-around and has previously demonstrated that she will not hesitate to increase the aircraft's power; and initiate a go-around. My analysis is that while she was stabilized on her final descent; the strike and resulting loud thud redirected her attention and caused her to lose focus during the most critical phase of flight. Following this accident; we will be conducting 3 lessons exclusively working on go-arounds; and go-around procedures to ensure this does not happen again. During those lessons; I plan to use all the tools at my disposal to continue simulating real world distractions.