I would start with a slide-show, an image is worth a thousand words ( I don't know if you are able to use this for the English class). You may continue about stories, people you may have known that have experienced drug abusation problems. If I were you I would not go directly to the happy ending, try to simly indroduce the story. As it is a persuasive speech you can also mention researches, surveys, use numbers (but not too much), give reasons, state even your own idea about drug abuse. Talk about common drug of abuse, sometimes people tend not to realise the state that they are, so try to explain firsty in general and then (as it is a class) you may develop teenagers/adolescents and drug abuse.
There you may also include the preventions tools and so going on the speech will be interesing and also real. If you decide to start with a story you may end the story by the end of the speech.
Hope this would be hopeful.
Talk about how the prolonged meth use can worsen someones appearance