First and foremost you are worrying about something of no consequence, two days in class will not stymie your efforts or set you back unless you choose to let it. The "magical" 64 ounces of water a day has no foundation in any research it is simple a mantra that has been repeated so many times it has taken on the guise of being holy writ. As for food intake there are several nutrition bars, Balance Bars and Zone Perfect for example, which have a proper balance of macro nutrients without excessive calories.
For me the more critical issue is the one of prolonged sitting. When you get a break stretch, especially the hamstrings, and walk around, do some squats jumping jacks, anything to get your circulation awakened. Do not worry what others may think, it is your body and your health so do what you need to do.
It is called WORK-ing out for a reason.
I said getting fit was simple, I did not say it was easy.
Cardio burns calories, strength work burns fat.
Eat well to lose weight, exercise to get fit
You can not build a six pack using twelve packs
Often when we seek a magic bullet for fitness we end up shooting ourselves in the foot.
"I think calories are little germs in food that all moms are afraid of" Dennis the Menace