(1) In Wang Jun's grades, 78 marks appeared the most frequently, twice.

So, the mode is 78;

Zhang Cheng's achievements are listed as follows in descending order:

75、75、77、79、80、80、80、83、85、86,

Therefore, the median =12 (80+80) = 80;

So, the answer is: 78,80;

(2)S2 =110 [(86-80) 2+(80-80) 2+(75-80) 2+(83-80) 2+(85-80) 2+(77-80)

= 1 10(36+0+25+9+25+9+ 1+0+0+25),

= 1 10× 130,

= 13.