HELP ME!!!!!

So I'm already in some deep shit with my parents. Just got a letter from my school basically saying that I have to get my grades up or else. 
Side note: my GPA is a 3.1 and it went down to my surprise to a 2.3 
I know I can bring it up the coming semester but I honestly believe that they are not hearing or going to believe it. 
What do I tell them???
Preferably a believable lie