How can I stop failing my science course?

Okay so my teacher just gave us our averages in science, and mine wuz 21%. I didn't understand why i got this cause i got an 86% on our last quiz. Then i found out it was because i didn't hand in some of my homework ignments. I was a very good student back in grade 8, but in grade 9 i'm startin to slack off. Is there any way I can get back on track? HELP ME???