I read this book many many times as ~ten to twelve year old in the late eighties.
My copy was donated to some charity at the age of thirteen by my very helpful sister (including my cherished copy of the Hobbit!). I've thought back to this book many times over the years and can remember the plot and characters quite well but have been unable to remember the title or figure it out with extensive googling.
Here is the plot briefly - If anyone can tell me the name of the book I would be in your debt!
The book starts with a young girl about 8-10 years old who has a fairly unpleasant mother who is in her late teens at most. The mother repeatedly tells the child she was only born so that she would be given a room to live in as opposed to a dormitory. Food costs, water costs - the child is constantly hungry. The only food avialable is some form of deep fried cabbage.
The kid goes to a school - this is run by computer and there is very little in the way of schooling - i dont think reading is taught -just he bare minimum social programming to get a kid to function in this horrible society.
There is a boy in the class - called Michael I think, who is a bit wierd.
At this point you begin to realise that everything is underground - only that the inhabitants have no concept of the surface.
This kid Michael claims to be from the surface and describes it as a land of milk and honey essentially. He was messing around exploring and fell into a mine shaft and after many days of going deeper and deeper he got scooped up and sent to school.
They decide to escape.
At this point from the description of the tunnels that they are in you suspect that the tunnels go all the way round the circumference of the earth. This isn't made explicit.
They spend days going up and think they are being followed. I'm a bit hazy on the next bit but they make it to the surface and just manage to escape being chased by some adult guy.
They get to the surface and everything is very nice, but people are living in domes as they think that the outside is poisonous, lots of food and all the conveniences of a future utopia. Only the people at the top have no idea that there are billions of people living beneath them.
The kids get picked up by the secret police who have been onto them for a while. They get given an orange as a test. Michael knows what it is and peels and eats it. the girl is suspicious at first then ravenously eats the whole thing, peel and all - displaying that Michael is a true surface dweller and that the girl is an underground person.
They get shoved out of the front door so as to not upset the balance of society and told to scram (or they escape-i may have forgotten that bit).
They tramp overgound for a few days and realise that they are still being followed by the adult tramp guy from underground. It turns out that he was secret police and meant to be keeping an eye on Michael, but he was so fed up with life that he tried to escape also.
In any event the three of them eventually get to Michael's home village/town where a few people are still living normally they way we do.
So any bright ideas?