Explanation:

Let the number of students in the class be 'x'.
Given if 1 is absent rest of people can be divided into 6 equal parts i.e (x-1)/6.........(1)
And given if 2 are absent rest of people can be divided into 7 equal parts i.e (x-2)/7.....(2)
From (1)&(2), we get
x-1/6 = ((x-2)/7) + 1
=> x = 37.