Yep. I'm sure that the inability to digest chitin is surprising for a lot of people. It would be like eating a corn cob for you and I. You'd get a bit of energy out of it, but you're going to pass a lot of undigested material (fiber). To be fair, snakes can go for a month without eating anything without blinking. This is litterally true because they have no eyelids (okay... terrible joke, I know). Anyway, they'll live for a month without eating anything and still show a bit of growth because they're using their limited energy reserves, and their metabolism slows. If, however, you compare their growth rate to the growth rate of snakes on a more nutritious diet (fish or rodents), you'll find quite a difference. We're not even beginning to discuss the lack of calcium, vitamins, minerals, and other essential fatty acids/nutrients that a diet of pure crickets would cause.

I also think that you'll be able to control (and describe) their living conditions if you house them in your garage, using lights on a timer and supplemental heating. It's a lot easier to say, for example, that they experienced daily conditions of 14 hours of light and 10 hours of darkness and day:night temperature variations between 85 & 75F than it would be to try and describe cloudy, sunny, rainy, windy, hot, cold, etc. conditions experienced outdoors. May I suggest that a sample size of two snakes per group is insufficient? If you have a huge range between two individuals, you won't be able to say if one of the individuals represents the norm or the exception. Hope this helps you develop a really awesome science fair experiment.
Rick