You have to re-examine your reasons for doing what you're already doing (school, working out, etc.).
Make a list of all the reasons that you're doing what you're doing.
Then, more important, write down all the things that are going to happen to you if you don't shape up. This is the "Alpo".
Now visualize them. Vividly. Really feel what a nightmare it would be if you dropped out of school, got fat and stopped working out, and so on.
Revisit this every time you're tempted not to go to class.
You're probably too comfortable. You have to be in pain to be really motivated.
And I know, that sucks. But fear, anger, and guilt are much more motivating than joy, satisfaction, and happiness.
Dan "Life is Suffering" McVicker