In April 2020 I bought a new KX 250 for my 15 yr old son ( this is his 5 the bike growing up ) He has been riding it regularly. This weekend we took him to take a lesson at a professional track near us here in Fla. After about 40 mins on slow riding in a small circle ( He was taking a lesson on how to properly ride, for racing) All of a sudden the back wheel locked up, at first we thought it was the brake, but found that, after reliving the pressure from the brake it was still locked up, and would not move even with the clutch engaged. Fortunately the instructor came over and suggested we push the bike backwards, after about 5 feet it came unlocked and seems to be alright now. What could possibly cause the transmission to do this? Any help or suggestions would be appreciated . Thank You