There are five people missing from seven people in each row, but in fact there are more than seven people in each row:

7-5 = 2 (person)

So the number of students in grade six is the common multiple of 5, 6 and 7, which is greater than 2.

There are no more than 250 students in grade six, so grade six has:

5× 6× 7+2 = 2 12 (person)