My recommendation is to stick it out.
Although the workplace might not have people your age, imagine the day when you graduate and would need to find a job. How would this experience (or lack of) set you back? In addition, what makes you think that your future job would have people your age? Your internship is a small taste of the 'real world' at large. Although it would sound harsh, it is time to toughen up and face the facts here. You have to learn to cope with undesirable conditions and excel regardless of your circumstances. In time, after you have demonstrated and proven yourself, you can then call the shots as to how things work.
According to a 2014 survey, approx. half of internships are paid and the other unpaid. But a cursory research into the reading doesn't support the notion that they pay well. In my experience, my first internships were paid minimum wage, it was only after I've developed a work history and marketable skills were I offered wages above minimum wage: $9.25 (hauling and sorting trash), $10 (research assistant), $15 (research assistant), $17.25 (application developer intern), $25.20 (industrial engineering intern). Knowing the value of my resume after graduation allowed me to better negotiate my starting salary at my current position, otherwise I would not have had the confidence to request a better offer.
You say that you are getting great experience, despite the actual work being dull. One way to make the experience more exciting is to ask your manager if you can shadow a senior engineer or another project during work hours such that you can get a better sense of the whole project. If you don't ask for these types of opportunities, you manager won't know that you are interested.
As an intern for UPS, my daily work was mundane, data-cleaning. But what I did was to spend my breaks and (with permission of course) interview senior-level staff at UPS and learn about their operations and overall project goals.
If you find yourself alone at work, focus on the work and make sure that you are bringing 100% levels of effort. Ask if you can collaborate in larger teams or start a project with fellow interns. This is also an opportunity to demonstrate leadership characteristics that few interns have the chance to. After work, you are free to volunteer, go to a park, participate in events, and socialize. You feel lonely as a result of your actions, seek to push beyond it.
At the same time, I have considerable doubts that you can just "go back to school" on a dime. Class enrollment is a drawn-out process, you can't just start a class in the middle of the semester/quarter.
At the end of the day, internships are a time to grow and learn on your own. No one is going to tell you what to do that will align with your interests. It is time to take charge and well... act like an adult.