In my opinion, just review the school handbook with respect to the gravity of offense for the act committed. If the handbook states that minor offense lang yung act na ginawa ng anak nyo, hinde pwedeng gawing major offense yun ng school, by simply stating na nagkamali sila. You have legal ground to fight for it.
However, if the handbook stated otherwise, then, thats a procedural glitch lang and the school have the right to increase the gravity of the offense as stated in the school handbook.
Now, if walang naka state sa handbook with respect sa act na ginawa ng bata nyo, then, the school committed abuse of its right in increasing the gravity of the offense. You have a legal ground also to fight for it.