When something is compulsory, it is required or must be done. In most states, it's compulsory for kids to attend school (or an equivalent homeschool) from age six to seventeen.
This adjective is from Medieval Latin compulsorius, "using force," from Classical Latin compellere, "to force." The Latin suffix –orius corresponds to English -ory, "containing or involving." Other English words descended from the same Latin verb are compel and compulsive.