I'm not a teacher but I would tell a child to do you best in school and when you're done find something that you (love) doing.
If you become an "expert" at it no matter what it is you can either start a business or be a highly paid consultant.
I would do my best to avoid being a "Dream Killer" or planting a negative seed in a child's mind that could stay with them for life.
In a free society there are multiple paths to success and reaching one's dreams. Sometimes you win and sometimes you learn.
I would not tell an average student they're doomed to mediocrity.
The most important thing is getting in the habit of doing your best!
That's all any of us can ask of ourselves and of each other.
Too many other factors come into play regarding success such as networking, appearance, personality, and timing. Never give up!