dell optiplex 790 BIOS WoL wont work!!

i have a classroom full of these computers and we need to be able to turn off and on these computers remotly on imperio consol. but nothing seems to work in the bios wit wake on lan and its getting anoying now, anyone have the same problem and how to resolve the issue we have tryed everything. and sometimes when it finaly works the computers seem to automattically turn its self on some how with out a magic packet being sent :/.