O.k. Most likely if you'd asked, they would have said to watch the demo rider.
In lesson 13, accelerating from a stop in a curve, did they have you shift, or rev it really high?
You need to be holding in the clutch while you are shifting. There shouldn't be a grinding sound. It's a ratchet system, so you need to click down, release, once for each gear. What bike is it again?
The other thing is to be going the speed that is appropriate for the gear your shifting to.
I think OP said CBR300