Do you have to go to art school to become an artist?

No you don't, but that degree in your hands makes a big difference when it comes to climbing the ladder of success. Or getting that job you wanted so much, just to find out the one with the degree got it before you.Besides all that, going to art school opens up a world in art you never knew existed.Yes you can learn without it. But I think you would be more likely to get a job working for a company if you have that degree in art. Unless of course you start your own business. My ex wife went to a community college for two years and won a full paid scholarship to a prestigious art school in Philadelphia. Maybe your teacher did not go to art school per say, but she had to have taken up art at her University to be able to teach it. You can major in art at a University.

No, you don't "have to". Art school will develop your skills and may make you more attractive to employers, but it's not required. ..

I agree with the 3rd answer, your teacher was using art school in the broad sense, including what is taught inside a university and you are trying to use it in a narrow sense of the more modern, extremely limited vocational school. What you should get from either place is a knowledge of a variety of materials and background information on what others have done. But there is no reason that you can not train yourself to do good illustrations, especially in a well defined (limited) technique, especially as you wish to do it "on the side" A problem you will have working independently is making links to authors or publishers that want their books illustrated and making your style known to a variety of sources. Art training will make them more likely to trust you going in and willing to look at what you produce.