those 'ejection poles' that you mentioned are not there to break the canopy. The top of seat itself has two canopy breakers that makes the hole for the rest of the seat to follow (it's only perspex that the seat/occupant goes through).
The canopy stays on the aircraft and the top of the seat makes the hole for everything else to follow.
On hawks or other aircraft, the canopy itself has explosive chord in it to make the hole which is part of the ejection sequence when pulling the handle.