If you mean hand bag (sac à main) it is masculin, if you mean just bag (sac) it is masculin too. There are no rules that I've ever heard of, and I've been in France since 1976, it all works by "sound", except for a few rules: abbreviations are always masculin (un PC, un IVG...)
A schoolbag is "un cartable" in French; cartable is a masculine noun.
masculine
La salade is feminine
"Sac l'école" is masculine in French. The word "sac" (bag) is masculine, so it determines the gender of the entire phrase.
In French, "croissant" is a masculine noun.
A schoolbag is "un cartable" in French; cartable is a masculine noun.
Feminine
masculine
La salade is feminine
masculine
feminine
Feminine
In French the country Mali is masculine.
In French, "croissant" is a masculine noun.
balle is feminine in french / ballon is masculine
"Sac l'école" is masculine in French. The word "sac" (bag) is masculine, so it determines the gender of the entire phrase.
It's masculine. Beef in French is le boeuf.