how come in geography classes they say mexico is part of central america?

Central America usually means all those countries in North America that are south of the United States. It seems that the USA doesn't like to be included with Spanish speaking nations so it calls them Central America.

I've always heard that Mexico is a part of NORTH America. Central America starts with the smaller countries just south of Mexico.

Since there is a North America and a South America, the "connecting" countries are generally refered to as Central America because they are in the center of the two. Mexico could be considered either one because the majority of it lies in North America, but culturally speaking, it has stronger roots in Central America. So, it depends on who you ask, and what they base their response on, geography, or culture.

YOU GOT:
MEXICO- WHICH IS PART OF CENTRAL AMERICA.
NEW MEXICO-WHICH IS PART OF UNITED STATES.

i´ve never heard that.

as much as i know we are part of north america (even if some racist mf dont like that) .

If that was mentioned in your class, to make clear that Mexico is in North America tell the teacher that Mexico is part of NAFTA North America Free Trade Agreement, signed by Canada, USA and Mexico. So, if these nations are in agreement, why wouldn't he / she be?. Central America begins up north in Guatemala and Belice and ends at the south in Panama.