Yesterday, I left a startup as a summer student to go back to school. I already really miss the people, friends, job, and workplace environment

You made the right choice if you weren't a founder. Your education is more important than being some guy at a startup, especially if that startup doesn't succeed in which case you'll be left without good job prospects since most companies don't want to hire people without college degrees.

It's great that you discovered you like the startup vibe. It's important to know what kind of work environment is a fit for you..Startups are addictive but that magic carpet ride can also crash badly. Take it for what it was - a great experience that taught you much and brought cool people into your life. Good luck with school!