Mandatory sex education lessons were introduced to secondary schools in England this academic year but not enough funding has been given to train teachers, campaigners said.
Under the Relationships, Sex and Health Education (RSHE) curriculum, teachers should discuss issues including consent, sexual exploitation, abuse, grooming, harassment and rape, and how these affect relationships.
But Lucy Emmerson, head of the Sex Education Forum, said the Government must fund proper training for teachers to help them deliver the lessons.
She said: “For teachers to... facilitate complex discussions.... does take practice. The resources being spent are not in proportion to the enormity of the job.”
It comes as research shows that staff in four out of five schools have not received training to deliver the new RSHE curriculum.
Children and Young People Now magazine revealed that just 4,800 schools have received training from government schemes to deliver RSHE.
Labour MP Sarah Champion said: “RSHE is the most important tool at our disposal when it comes to tackling abuseand addressing harmful attitudes.”
It comes as around 200 people held a vigil for Sabina Nessa in Eastbourne, where the man suspected of her murder was arrested.
Teacher Ms Nessa was killed in Cator Park, Kidbrooke. Koci Selamaj, 36, has been charged with her murder and has been remanded in custody ahead of a plea hearing on December 16.
A Department for Education spokeswoman said the Government has invested more than £3 million to support staff to teach the new RSHE curriculum.