koolis käima means go to shcool regularly
kooli minema means go to school once
examples
mine kooli - go to school
he goes to school everyday - ta käib koolis iga päev
hope that helps
I guess it is like in Russian there is "idtii v shkolu" (=kooli minema) ja "hodit v shkole" (koolis käima) (sorry, but my keyboard doesn`t have russian alphabet)