This is a tough one, high school doesn't really offer particular classes to teach or reinforce phonemic awareness even though it is known that it is the leading cause of children reading poorly when entering high school.  It just doesn't seem to covered very well in elementary school (where it should be).  I have included a few sites that you may find helpful to work on at home, some seem a bit childish, but it does cover the material and sometimes we have to go backwards to go forwards.  Give these sites a chance and best wishes!