According to the known data information in the statistical table and the structural characteristics of the composite bar chart, the statistical table is completed as follows:

(1) 45+42+60+52 =199 (person);

A: There are 199 girls participating in the mountaineering competition.

(2) Number of students in Grade Three: 55+45= 100 (person);

Number of students in the fourth grade: 58+42= 100 (person);

Number of fifth-grade students: 65+60= 125 (person);

Number of sixth-grade students: 60+52= 1 12 (person);

In contrast, the number of students in grade five is the largest, and the number of students in grade three and four is the least.

Answer: The fifth grade has the largest number of students, and the third and fourth grades have the smallest number.