Felix Mendelssohn wrote twelve string symphonies between 1821 and 1823, when he was between 12 and 14 years old. For his mature symphonies, see here.
The string symphonies are written for a string orchestra. String Symphony No. 11 also contains percussion (timpani, triangle, cymbals).
Most of the string symphonies were composed in three movements, with the exceptions of nos. 7, 8 and 9, which are in four movements, no. 10 which is in one movement, and no. 11 which is in five movements.
Mendelssohn also created an arrangement of this symphony that included wind, brass, and timpani.
Mendelssohn also wrote a movement for a string symphony in C minor, known as the "Symphoniesatz".