Answer:
The student would definitely weigh lower than 71 kg when the student rolls down the incline on their skateboard. This is because as he goes down, there will be air pushing against him, and though it isn't enough to fully lift him, the air pressure is enough to decrease the student's original weight. It mainly depends on how steep the incline is; if the incline is just enough to get him going, then the bar will show just a bit less than 71 kg after a while. However, if the student goes down a much steeper incline, say something like just a bit further out than a wall at 90 degrees, then the student will most probably have a much less weight than 71 kg.
Kinda long, but hope it helps.