We were there two weeks ago and two of our friends went into the intermediatte classes and thouroughly enjoyed it.
The enjoyed if from the point of view that the instructor not only improved thier ski technique, but also showed them slopes they may not have ventured to, aswell as passing on stories of serbian life and giving geography lessons on the surrounding area.
Also, if you are hiring equipment, when pre-booking together with ski school you get your lift pass free. So it only works out about an extra tenner for ski school.