There is definitely a time and a place for negative corrections, violations of school protocol for example. But overall I've found that teaching students what to strive for gives them a better understanding of their progress. This is especially true for teenagers who I find don't generally react well to constant criticism.
This idea is a break from a lot of traditional martial arts styles. Many old school teachers are stern and punitive in their method. While I keep my training sessions tough I find positive corrections to be a better fit for modern western students.