I taught for 31 years and only had a summer job once, when I was about 25; I worked at a department store. They hired me on a permanent part time basis and then I just quit at the end of the summer. I hated it. Even then, after having some modicum of control over what went on in my classroom and feeling like a professional, I just couldn't handle my supervisors nit picking about things that were essentially stupid. And, the job was boring. Teachers make hundreds of decisions a day in the course of their work and I just didn't like being in a job where all of my decisions were essentially controlled by someone else. So, I guess I agree with your friends.
A few years later I became a certified fitness instructor and taught in gyms before school and after. So, for about ten years, I did this in the summer and often volunteered to sub more. That was fine and I enjoyed working with adults!
Have you considered tutoring? You choose how much you work and the going rate around here is 50-100 dollars an hour. If you don't want to bother developing a client base, you can work for one of the bigger agencies, although you don't get as much money.