intel 10/100 pro ve WOL issue

Hi

At work we have about 250 pc's with intel 10/100 pro ve on board nic
card with the ability to WAKE ON LAN.

My problem is if i do a start ==>shutdown the next time i try to use
wake on lan it will work ok


but if i shut down with the power button, and if i then try wake on
lan it will not work.

This is a problem because I work in a school with 8000 plus students
and they all use the power button.

what is wrong with the WAKE ON LAN.

all pc's have windows xp pro