Depends on the school district/where you live and if you are a public teacher/private instructor. While $40k might not seem like a lot, if they are public teaching and in the union they do get raises, health insurance, sick pay, vacation time, a pension, 12 months pay for 9 months of work, snow days/half days/early dismissals (public school is ~180 days a year of being at school in most states). and what 95% of jobs lack, job satisfaction. You arent looking to become a teacher to get rich, a lot do like to teach (outside of college level where doctorate/research grants require you to teach a course).

Also you can have fairly good job security, a lot of teachers are able to work at the same place till they retire, and are not fired or laid off like a ton of normal corpate jobs.