My child’s religious education teacher recently told the class that french kissing is a mortal sin. Is this correct?

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 (CCC 1857, 1859). 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.