CORRECTED: The reply below is ok. We also very often say branche (with an accent acute over the e) for cool when used in this way. So the answer to your post would be to replace teh word cool below with branche.
C'est pas cool
Ce n'est pas cool
'chouette' or 'super'
Soyez cool
c'est cool
thats kinda it, junior IT STAYS THE SAME
Tu es cool.
Just say oh thats cool and forget unlesss its about a boy
je suis cool !
Hyper-cool
C'était cool
je suis tellement cool
T'es cool, mon pote.
"Il est super" or just "Il est cool."