Easy does it! The Smart Scholar Method

 
It clear that Harinder and Surinder are of different genders, as, otherwise the answers would have been identical.

Compare 5/14 and 3/8. We see that 5/14 = 20/56 and 3/8 = 21/56. Obviously, Harinder is a girl and Surinder is a boy.

If it is not ‘obvious’, and further explanation were needed, it can be inferred that the person who says that the lesser fraction, namely, 20/56 (and not 21/56) of classmates are girls is a girl herself [and the person who says 21/56 (and not 20/56) of classmates are girls is a boy]. Therefore Harinder is a girl and Surinder is a boy.

In case a mathematical solution is required, let the number of boys and girls be b and g respectively.

If Harinder is a boy and Surinder is a girl, Harinder’s statement means ‘g/(b + g – 1) = 5/14’  and Surinder’s statement means ‘(g – 1)/(b + g – 1) = 3/8.Subtracting, 1/(b + g – 1) = 5/14 – 3/8 = ( - ) 1/56 which is not possible.  Therefore Harinder is a girl and Surinder is a boy.

Further research:

As Harinder is a girl and Surinder is a boy, Harinder’s statement means ‘(g – 1)/(b + g – 1) = 5/14’  and Surinder’s statement means ‘g/(b + g – 1) = 3/8.Subtracting, 1/(b + g – 1) = 3/8 – 5/14  = 1/56. This means (b + g – 1) = 56. Substituting in g/(b + g – 1) = 3/8, g/56 = 3/8 or g = 21. Therefore b = 36. The strength of the class is 57.

Sanity Check:

What Harinder, a girl, says means (21 – 1)/(57 – 1) = 20/56 = 5/14. What Surinder, a boy, says means 21/(57 – 1) = 21/56 = 3/8.

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