Allow access to selected web sites only I have a lab of MacBook computers. How can I limit the internet access to only learning sites I choose (like Study Island or Plato)?
This sounds like you need something that will limit the MacBooks to an allowed "white list" of web sites, which you set up.  ("White list" is the opposite of "black list", which is when you set up a list of web sites that are not allowed.)

You might be able to use the built-in OS X Parental Controls system... go to Apple menu > System Preferences > Parental Controls.

Apple has some documentation on Parental Controls here, which includes the picture below: see the "Allow access to only these websites" button.