Some thoughts that might be of help I hope...
Kids/young adults that age aren't so interested in taking care of themselves - they're into a bit of risk and don't feel susceptible to anything - they're indestructible!
So I'd suggest you gear the presentation towards information on how they might help a friend who needed their support. It can give them the insight/awareness they need without feeling personally challenged.
I'd cover the main topics of:
What is mental health - What is mental illness. Do the "1 in 4 people have mental health issues - think of your 3 closest friends and if it isn't one of them - it must be you"
Most common lifetime issues - eg anxiety, stress, reactive/adjustment depression, phobias
Most serious conditions - eg the psychoses
Most common serious adolescent issues - eg suicide, substances, stress and I'd also add some PTSD here.
..and present each topic with recognising it by understanding some of the key processes and how to support a person with such a condition.
I'd also add 'stigma' into the equation and try and dispel some of the urban myths.
How long have you got to present for?