If anyone knows a bit more about this story, could they let me know please? It's not entirely clear whether the pupil did actually assault the teacher.
There are plenty of Heads and School Governors out there who wouldn't expel Hitler. If teachers had a policy of always refusing to teach a child who had assaulted one of them and insisted on involving the Police every time, then these incidents would be taken more seriously. It would also help if their Unions took out Private Prosecutions when the CPS can't be bothered to do so.
None of this will ever happen, so these events will continue to be brushed under the table.
(I also ended up being brushed under the table last weekend at the Iraqi Beer Festival)