Nowadays it's a standard for every supersonic military jet to have ejectable seats! (Otherwise it's a jet for corn pollination) Regarding L-159, look on a picture http://upload.wikimedia.org/wikipedia/commons/1/12/CIAF_2009_Alca_L-159_2.jpg [^]
Do you see that beige colored block behind the seat? This tells us that the seat is ejectable!
Edit:
This was partially fixed, at least pilot is being thrown up after ejection, giving him a safe landing altitude. Watch current state of this issue --> http://youtu.be/wvL4MgR719Q
The next step is getting rid of halo and simulate real electable seats with deployable parachute.