if b stands for boys and g for girls

notice that

b+g=30

also

b/2+g/2=15

if you say half of the girls (g/2) sit with boys this means g/2 is a pair number

but as b/2+g/2=15, b/2 cant be a pair number so its not possible.

edit:to explain better this

if you say half of the girls (g/2) sit with boys this means g/2 is a pair number

this is because g must be pair (cant cut people in half)

and as half of the girls sits with boys and other half sit together in groups of 2

g/2 people sit in groups of 2, so g/2 must be also pair