I am going to guess that you already know how to enter the accelerative learning state. 3 to 1 countdown. So rather than explain the Silva Method Awake Control I'll give a brief method you can use, using what you've learnt from the photoreading system.
The reason affirmations are said just prior to opening your eyes for photoreading is because you are talking to your inner mind most directly with an internal focus and the inner mind tends to follow through. Thus making them most effective.
So instead of preparing yourself to photoread a book you prepare yourself to remain alert during class. You enter the accerative learning state in preparation for being focused in the class. 
You modify the affirmations to suit. Tell yourself that you want to remain alert in class. So you would state an affirmation like; I am taking xyz class while I am in in this class I will remain alert and focused on the information being presented in this class. I will understand and remember all the important facts that will help me get my best passing grade for this class. It helps if you say your affirmations three times also limit them to 2 or 3 concise statements. Say the affirmations a second time while you pick up your tangerine. Say it a third time when you open your eyes putting the tangerine in place. If you find your mind wondering during class or the tired feeling coming over you, just smile and reposition the tangerine.
Some things about affirmations. State them in the present and state them in the positive. EG Say I will remain awake and alert, rather than I won't fall asleep. You need to specificly tell your inner mind what you want to move towards.
While the awake control from the Silva Method would help you to remain awake it would do little to help you participation in the class. Thats why I presented a modified method that I think would suit your needs better.
Alex
