Sunday school has been around for a long time. 
Yes, i've never been in one but heard it can help pupils that are a little behind.
its more about Christian educations, usually for children
Do you think it's a value added activity on a sunday?
It can if it provides actual education and a quality learning environment for Christian theory
What is the drawback of going to Sunday school?
They were first started to provide real education for working children in England which is a good thing but now the sole focus is on Christian belief, which i find might be limiting or a drawback
I can see that being limiting. I don't think nowadays kids have any other way of learning about Cristian faith in such a detail than Sunday school.