Citizens of the United States or Canada who are planning a trip to Mexico will need to carry a passport or WHTI-compliant travel document. A passport is necessary for everyone entering Mexico by air. Travelers entering Mexico by land may not be asked to present a passport, but will need to present one upon return to the US or Canada, so be sure you have it. The passport requirement is waived in some cases for minors, notably, school groups that are traveling together. Since requirements vary from country to country, the best thing will be to contact your nearest Mexican consulate and inquire with them.