A snake can easily go 1 week without food. And if it's an well established animal, 2 weeks is no big deal either. Feeding a larger than normal diet right before you leave should help. But remember garters don't really expand as well as a ball python when being fed larger prey. Is there no one who can feed your snake while you are gone?
The 3 month summer break will be tough. Is there anyone who is staying at school off campus during summer that can care for your snake? Or perhaps find a local keeper or reptile shop to board your snake for those 3 months.