Honeybfly, I have the same problem. Whenever I open my books/notes to study, I want something to snack on. Now I'm in grad school, and I think that the habit came from my undergrad study habits--ordering a pizza or midnight runs to Taco Bell during exam week, etc.
If I'm truly hungry, I'll have some string cheese or pork rinds. If I'm not and just feel like having something in my hand/mouth, I avoid the extra calories and drink decaf coffee, green tea, or Diet Rite.
The urge to eat while I'm studying is, however, starting to fade. I think that once you get out of the habit of grabbing something to eat every time you open the books, you'll stop craving it.
By the way, I didn't gain the freshman fifteen....I gained the sophomore 25!! And the reason is that I ate to comfort myself whenever I was stressed about school.
I hope that this helps.