In most instances, once a decision has been made, it is locked in (the exception is the house footprint selection, which is not locked in until the permit is approved). If students want the option to change items, the best way to achieve this is through select saving.
The game always opens to the last place saved. Say your students save at the beginning of the Building Phase. If they choose their building materials and construct the walls, but decide they want to change the building materials, they should select "Quit Game" from the Menu page, and then select not to save changes. When they re-open the game, it will open at the last saving point - before they built the walls.
The same is true of the Landscaping section. If students save right before they begin placing trees and other landscaping items, they can go back and start again if they don't like the finished product.
In version 2.0, students also have the option to "Save As," so they can save a game at a certain point under another name, continue on with the present game, and select the "saved as" game if they decide they want to try again from the earlier point of play.