I am struggling on the reading and writing portion of the SAT and am unable to get my desired score. I have practiced Khan Academy and are there any websites (Free preferably) that highschoolers use to study these SAT concepts? 
I urge you to try the Erica Meltzer reading and writing books. The reading book provides great strategies and examples based on each type of question you might come across. The writing book summarizes all the gramatical rules you will need. I believe these two will be the only guide books you need concerning reading and writing.