As an educator, you can choose whether to create a standard word count goal for all students in your class vs. creating or allowing individual goals. Though it's important to challenge students to work at their own capacity, writing a lengthy novel isn't the ultimate objective. Placing students in a position to reach their word-count goal is a huge confidence-builder!
As you consider this issue, think about your students' writing experience, their ages, schedules, and enthusiasm for NaNoWriMo. Their goals should stretch them in some way, but still achievable; inspiring, not discouraging. For some people, that means a goal of 1,000 words - for others, 100,000! Look to the Word-Count Goal How-To for more information and a suggested word-count guide.
An alternative idea: we sometimes encourage young writers to adjust their goals throughout the month. The very process of writing itself often reveals a student's natural writing pace. Instead of winning on word count, you could allow students to win based on consistency.