Can they lower my grades?

I received a letter in the mail that said i have missed 10 days throughout the school year and that consequently, i am in danger of falling below grade level. I am a straight A student and i always complete the work I missed during the days I was absent. My question is: Do they mean that they will lower my grades or that if i miss one more day then my grades will be lowered? Also, this is the first letter I have received regarding my absences so can they really lower my grades just like that? Should I take this letter as a warning letter?