Swedish activist Greta Thunberg on Tuesday joined teenagers in France protesting what they claim are poor conditions at high schools. 

The 23-year-old activist, best known for outspoken support of Palestine and harping about what she describes as a "climate crisis," unexpectedly joined another day of marches in Paris, the Associated Press reported. 

The teenagers are protesting run-down schools, chronic teacher shortages, overcrowded classrooms, and an unpopular university admissions system. Thunberg said at the rally outside a Paris high school that the students are "fighting back." 

The protests have reportedly been unruly, resulting in mass arrests and injuries.