Teachers in general work far, far more hours than they are officially "required" to. They simply don't get paid for enough time to actually do all the lesson prep and marking necessary to do their job.
Schools don't operate for about 3 months per year. If you're looking at hourly rate, teachers are on less than three quarters the yearly salary of a full-time position at the same rate.
As was the point of this meme, teachers are often expected to spend their own money on things for their classrooms. This can range from party stuff to paying out-of-pocket for print-outs of worksheets given to students, to literally needing to buy pencils for students to use.
Teaching is not a low-skill job. It requires a university degree, during which time they likely supplied many hours of unpaid labour in the form of "prac" work experience. Early pracs might be mostly learning experiences where the student teacher isn't providing much value, but towards the end of their degree teachers will often literally just be the ones responsible for teaching the class.
It's, like, an important job. Arguably the most important job in our society, up there with far-better-remunerated roles like doctors and civil engineers in terms of doing things that are fundamentally important to keeping society running.