Focus on the positive to avoid discouraging students.

Like 

Sometimes the path to becoming a computer scientist is tough. For you it might have involved facing discrimination and a lot of hard work! Make sure to focus on the reasons that you stuck with CS rather than sharing the challenges you overcame. Any stories that make you seem super-human can make success in CS seem unattainable. For young students there is no need to describe challenges you’ve faced.

Example: "My favorite part of my job is..."

See this and related tips as a Tip Sheet: http://csteachingtips.org/Tips-for-classroom-volunteers

Other Tips By: