Go through the entire education interview until you reach a screen titled "Your Education Expenses Summary". Click edit next to the student's name. That should take you to a screen “Here’s your Education Summary”. Click edit next to the section you want to change; tuition, in your case (click edit next to the school name).
Verify that you entered the box 1 and box 5 amounts correctly.
If that's not it, click the scholarship section and make sure you didn't enter additional scholarships. In particular, loan amount should ne be entered.
If all else fails, click delete next to the student name at the summary screen, and start over
