This is book number 6 in the Capitán Calzoncillos series.
When Dav Pilkey was a kid, he was diagnosed with ADHD and dyslexia. Dav was so disruptive in class that his teachers made him sit out in the hallway every day. Luckily, Dav loved to draw and make up stories. He spent his time in the hallway creating his own original comic books -- the very first adventures of Dog Man and Captain Underpants.
In the second grade, Dav's teacher ripped up his comics and told him he couldn't spend the rest of his life making silly books.
Fortunately, Dav was not a very good listener.