There are 13 students in the class. p = probability
p 1st girl is 6/13
p 2nd girl is 5/12
p 3rd girl is 4/11
p 4th girl is 3/10
p 1rst boy is 7/9
p of 2nd boy is 6/8
p of 3rd boy is 5/7
p of 4th boy is 4/6
p of 5th boy is 3/5
p of 6th boy is 2/4
I multiplied all of the fractions above (cancelling wherever I could and again to check my work I reduced fractions that could be and cancelled again) and I got
1/572