I wholeheartedly embrace the idea of an online census. It's faster, easier and for the National Statistics Institute - cheaper. And already more than 1 000 000 people have completed the online form. I say it's a pretty good start and a definite improvement. I'm still not clear as to why foreigners and Bulgarians currently in other countries can't use the online form, but I hope this will be possible next time. 
You should definitely ask around at school if they're gonna organize something. 
I don't know when the census people will come looking for you, because last time I didn't pay much attention to that. But it's likely that they'll put some sort of notice on your building door the week before, with the date and a specific time. At least that's how it goes when they come to read the water meters and and heating.