I'm a college student who has been working at a remote part time internship for about 1.5 years, and I've found that I just don't have the time anymore during the school year. However, this is a good job for a college student, and I would like to be able to pick it back up in the summer. How can I write the "resignation" email to help leave that option open?
There is, of course, nothing you can do to guarantee you will have the option of returning. There is an inherent risk that the employer will replace you with someone who is available all year. The fact that they kept you for 1.5 years suggests that it is an all-year job, not just a summer job.
The best you can do is to write a resignation letter expressing positive thoughts about the job, regret that you cannot continue it during the school year, and your hope that you might be rehired in the summer.
I disagree with Patricia's answer.
Rather than simply write a resignation letter, I would talk to your supervisor/boss and explain that you need to focus on your studies. I assume they are already aware of your status as a student, so they shouldn't be too surprised.
Offer them a timeframe when you should be able to return.
Obviously, they can just say "well, it's time to call quits", or they can agree. They probably won't hold the position open, but they might find something else for you when you return if they have been impressed with you.
You are not going to lose anything by asking.
Just be prepared that situations change - they may not be able to place you in the future. You might change your mind later - just make sure you give them plenty of notice if you do.