Presumably, your teacher meant that french kissing is grave matter, and if done with full knowledge and full and free consent of the will it would be a mortal sin. Here are the prerequisites for mortal sin from the Catechism of the Catholic Church
For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent" (CCC 1857).
Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin (CCC 1859).
Presumably, your teacher also intended that french kissing is grave matter for the unmarried, particularly for the teenagers whom he or she was addressing. For married people, french kissing is permitted as part of the marital relations they share. For the unmarried, especially for the young, this type of kissing, which prepares the body for sexual relations, is inappropriate to the chastity expected of single people and could possibly constitute grave matter. Whenever a single person is uncertain whether the physical affection he has shared with another person is appropriate to a chaste relationship, he should ask his confessor who will be able to advise him.
by Jason Evert
If You Really Loved Me
by Jason Evert
Pure Love Club