well we can set up this proportion. 
2 boys 
---------
3 girls 
Now lets add this together to equal a smaller proportion class. 
2 boys + 3 girls = 5 students. 
We need for their to be 30 students. 
So what number can we multiply them by to get 30 students in all when added. 
Well we can divide 30 ÷ 5 = 6 
so multiply each number. 
2 boys x 6 = 12 
3 girls x 6 = 18 
these two numbers add up to 30 and when this fraction is simplified it is 2/3
So your finally answer is....
12 boys and 18 girls. 
Good Luck! :)