Oh boy. You don't know what you're asking for. Ha ha. Well, here is my amateur take on it. This is a delightful story, actiony, love the punch at the end. Oo, so glad I wasn't you, sounded very unpleasant. The pace flows pretty well.
I went through and dissected everything I could think of, just because I have nothing better to do at the moment. Take my advice with a grain of salt; use what you like, ignore what you don't.
(This opening works perfectly for its purpose here, but if you were to use this as a short story anywhere else you might want to change it to ?In third grade I?? or ?When I was in third grade I?? etc.) This one time in third grade I went over to a friends (friend?s) house... and as we were getting off the bus, he turned to me and said "Are you ready to run?" At first I was confused and stared at him blankly (I am a fan of adverbs, but the pros really look at them. Using too many is automatic amateur stamp. You have six in here. Use them when you feel you must; avoid them when you can. It?s hard, but an effective way to get around using them is to find a verb that does the job.) so he quickly said (this one, for example, could be hissed, uttered, cried, whatever.) "Just run as soon as the bus door opens."
I learned why we were running before the door had fully opened, right as I looked up to get off the bus when from the trees close by came a hissing sound and then a sudden pain on my forehead that felt like a bee sting. (This sentence is long. If you read it out loud it kind of drags on and leaves you confused. Here?s a suggestion: I looked up as the bus doors opened. Something hissed from the nearby trees, then a sudden pain on my forehead like a bee sting.) A few chuckles echoed from the trees and I instantly knew (to get rid of this adverb try replacing ?instantly knew? with something like ?realized? or ?understood?. The reader will still understand that it was a quick thing.) the threat. OLDER BROTHERS! I ran faster than I had ever ran (run) before but it wasn't fast enough. Following the blow to my forehead, (You can cut this first part out so the sentence looks like this: ?Pellets hit my hand, my knee, a couple bounced off my backpack, and then one got me, right in the back of the head??)pellets hit my hand, and then my knee, a couple bounced off of my backpack, and then one got me, right in the back of the head...
The impact startled me and I tripped on a rock and fell head first into the bushes next to the front porch of my friends home. (This one is long too. During the more exciting parts, sentences can be shorter. It gives a story a staccato effect, helps the sentences move at the same pace as the story. Put a period after ?startled me?. Here?s a suggestion for the next line: ?I tripped on a rock, falling head first into the bushes next to his front porch.?) Dazed but still somewhat startled(,) I turned and looked at my friend as he ran through the door(,) slamming it behind him,(.) (The sound of pellets bouncing off the door started to bring me back to reality.) the sound of pellets bouncing off the door starting to bring me back to reality.
The pellets shifted from the door and then started again to bounce off of me as I pulled myself to my knees and crawled up the steps. When I was close enough to grab the knob, the door opened slightly (to get rid of ?slightly?, ?the door opened a crack?). It all happened so fast it was like a blur,(.) I thought at first it was my friend coming to drag me into safety, but I was wrong. His German Shepard (shepherd) quickly pounced (Pouncing is a quick action. You don?t even need ?quickly? here.)on me and knocked me backwards down the steps (?on me, knocking me??). "Sick Him, Sick Him" came from the open door(,) but it wasn't my friend's voice I heard. The dog growled and barked but never attacked me. Then the laughing starting..(started,) from the bushes, from the doorway, and suddenly all 3 of his older brothers were there (you can cut ?there?) helping me up and into the house. Inside the house my friend was stuck on the stairway in pain from a massive wedge (wedgie) he (had?)just experienced from the oldest brother hiding inside, waiting if either of us happened to escape the pellet attack.

Can you believe I did all that?