In most situations, consistency is very important. However, you minimize risk by having a solid, defensible, business-related reason for granting a request in one case but not in another.
Situation: You've granted one employee part time to go to school, and another employee with poor performance also asks for the same thing.
If you believe, based on the employee’s performance, that she could not satisfactorily complete the essential functions of the job with part-time hours, you could deny the request based on performance. Just be sure that you have documented the performance concerns and that you can reasonably explain why part-time hours would worsen performance. You will also want to be certain that the employee whose request was granted does not have any similar performance issues. Again, consistency is usually the best option. You might benefit from coming up with some type of standard to address questions like this in the future.
A final note: if the underperforming employee were to improve her performance in the future, it would be in your best interest to re-review her request.