Two F's and alot of excuses

Sterling has two F's this trimester. I don't think he can dig himself out of this one. These are solid F's. One in math and the other in history. He has many missed assignments in math and a couple in history. When I discovered he had TWO F's, I grounded him.
I took away what hurt the most. Electronics. In addition to this, I asked him to start using his school planner. I did choose to let him go outside, which is a first when grounding him. My thought process was let him get exercise. He can't go into other kids' houses but can ride his skateboard around.
Once the groundwork of the grounding had been laid, he ran to the door, with skateboard in hand.
"Stop" I barked. "What time are you coming home?" I asked.
"Four." Said he.
"OK, four. Not one minute past though." I continued.

He was two minutes late. I chewed his ass. He had the stones to blame his tardiness on a U-Haul truck that was backing up. I asked him if going around the truck...using the grass to walk on, was a possibility.
"No." was his response.
Fast forward 5 days. I decided to let him out again.
"Don't make me regret this decision." I told him. "Be back AT 6! Not 6:01...not 6:02...SIX"

Did he come home on time? Early? Nope. Not my son. Not the king of unorganization. Not the king of excuses. Not the king of not doing what his dad says.

6:07 he came in crying. I didn't even turn to see what was wrong. BECAUSE NOTHING WAS. IT WAS GOING TO BE AN EXCUSE AND I DIDN'T WANT TO SEE ALL THE CRAP THAT WAS GOING TO LEAD UP TO HIS FICTICIOUS STORY.

He claimed to have started home at 5:50. As he was only two minutes away, this should have been plenty of time to get home. Halfway home he claimed to have fallen and
REALLY hurt himself. I asked to see the "wound". Shocking...there was no scrape, bruise, cut or blood.
"YOU DON'T BELIEVE ME?" He said.
"Nope." I replied. "Why should I? You have given me no reasons in the past to believe you."
At this point he thought about acting mad...but I think he thought twice about it.
I originally grounded him until he got his grades up. His trimester ends on Wednesday, three days from now. That means that his grades automatically go to A's. The beginning of each trimester EVERY kid starts with straight A's.
Since the end of the trimester has put an end to the grounding, and not his effort...I'll need to figure a way to have him earn his freedom.
For now, "outside time" is gone. No longer will he be grounded for his grades. Now he's grounded for not being able to get his ass home on time.