Crickets will shorten the lives of your garters. I suggest you get rosey reds, a garter-edible fish that is also available at Petsmart and almost always available. In addition, Petsmart carries nightcrawlers. I find them more expensive than my local bait shop (where I buy minnows and nightcrawlers), but they will give your snakes better nutrition.
For a science project, you might take two snakes of approximately the same weight, age, and sex, and weigh them weekly, with one on the diet I have described, and one on the cricket diet.
If you have one of your students do that project, we would all love to hear the results!
(My father taught Science for 30 years on the secondary level.)
