It means being behind on your classes average grade. So basically means you are having problems with learning !

I could be an idiom, which means, SUCH PERSON IS NOT CURRENT OR DON'T HAVE CURRENT NEWS (KEEP AWAY FROM A SECRET)

If, however, s/he actually said that s/he dropped behind IN the class, then it means that s/he must catch up reading the assignments, completing papers, or finishing a project.