A very helpful series that really explains the Classical style of Mozart is called "Succeeding With The Masters: A Guide to Practicing and Performing the Works of Haydn, Mozart, and Beethoven". It's compiled and edited by Dr. Helen Marlais and published by FJH Music Company. There are two books, and once students are finished with them they will be well equipped to start the easier Sonatas by these three composers.
In the first volume the Mozart pieces are (in order of difficulty):
1. Minuet in F (from Maria Anna Nannerl Mozart's Notebook)
2. Minuet in C, K 6 (This piece, composed in 1763, was originally written for violin and piano)
3. Minuet in C (from Notebook for Maria Anna (Nannerl) Mozart.
4. Air K 15qq
5. Minuet in F, K 2 ( the six-year-old Mozart created an entire piece from one rhythmic motive, which is a three-note slur)
6. Andante and Maestoso, K 15 oo
7. Minuet and Trio, K 1
8. Allegro Moderato, K 15 qq
9. Allegro, K 3
In Volume 2, the Mozart pieces are (in order of difficulty):
1. Klavierstuck in F, KV 33B ( This "Piano Piece" was written at the beginning of October 1766, in Zurich, Switzerland, when Mozart was 10.)
2. Andante in F, KV 6 (This piece was probably composed in Brussels, Belgium, in October of 1763, when Mozart was 7).
3. Minuet in F, KV 6, Minuet II (This piece was written on July 16, 1792 in Salzburg. It is likely that Wolfgang's father wrote the bass line and had his young 6-year-old song complete the melodies to the the harmonies.
4. Little Funeral March, KV 453 a (This funeral march was composed for "Del Signor Mawstro Contarpunto." It was intended to be a joke, and Mozart wrote this when he was a grown man, probably for one of his pianoforte students. It was composed in Vienna, Austria in 1784.
5. Moderato in F Major, KV Anh. 109 b, No. 1 (15 a) (This piece is part of the London Sketchbook Mozart composed during the time he and his sister were giving recital performances in England)
6. Adagio in C, KV 356 (617 a) (This piece was written in Austria, for an instrument called a glass harmonica. He wrote in in 1791, the last year of his life.
7. Presto in B Flat Major, KV Anh. 109 b, No. 9 (15 II) (This is also from the London Sketchbook. )