Conquering my Arch Nemesis

Planning for some people comes natural, not so much for me. Planning is one of the most difficult parts of my job. Each day I must develop lessons for 4 different classes ranging from pre-algebra to algebra 2. The problem I have is I never feel like I have enough time to make each class the best it could be. Some days I spend an hour planning for one class, it is just not feasible to spend that much time on each class every day. Currently I am not really happy with the way I plan my lessons and units. I attempt to set a schedule/outline for each marking period (6 weeks) but I am not able to stick to the schedule because I am not sure while scheduling what type of background knowledge my students have. Perhaps next year I should create a test to determine what type of background knowledge/skills my students have. I currently use the method of pretesting but the problem is many of my students lack any sort of skills to show any useful information, the majority of them guess on all questions. I feel that the pre-req. test would provide more information about background skills than any standard related pre-test. Using this method I would be able to develop appropriate lessons based on the skill levels of my students. While lesson planning is not my favorite part of teaching I realize its value and strive every year to improve upon what I have done in the past and look forward to new ideas in the future.