Children of illegals are educated in this country; upon graduation should they be deported or allowed to stay?

Let me be clear because illegal immigration is such a hot button issue these days.  There are no easy answers.  However, I do believe that illegal is illegal!  And, if you want to become legal, there should be some steps that illegal immigrants can take that would help them get on the path to legalization.  I don't believe that adults who have entered this country illegally and have been productive citizens for 20 years should just be kicked-out of the country.   As for criminals who come to this country illegally, they should be sent back to whatever country they have come from.  

In Helen Thorpe's book "Just Like Us," she chronicles the lives of 4 Hispanic girls--2 legal and 2 not legal-- living in Denver. It delves into the obstacles they face being friends and the obstacles they face due to the illegal status of 2 of the girls.  The law says when a child is brought to this country by an illegal, they too are illegal.   So, if we (society) are constantly pushing kids to stay in school, I have to wonder what is the incentive for the kids of illegal immigrants to stay in school and get good grades when they know they'll be deported once they graduate?

Here's another question:  How can we spend so much money on educating children who have been brought to this country illegally and then say "...you must leave after graduation"?  American taxpayers are getting nothing on their investment of time and hundreds of thousands of tax dollars spent educating these children.  If a person--any person--seeks to make restitution and demonstrates good citizenship, shouldn't we help them?  There has to be a better way.