First sentences
To be honest, not everyone will judge your story by the first sentence, some might give it a page before they're sure, some might even read a whole chapter but it's still important to get your beginning exactly right because if you've got a good beginning, people will read through the not so good writing and hopefully come to another section of interest that keeps them reading again. You say you know that this part is boring and that your real plot comes later but that's not good enough because people would read this and be turned off. You have to have hints of something else right from the start and you have to have something vaguely interesting at the start. But back to that first sentence then, let's take a look at yours:
One shot to the face was all it took to send him sprawling onto the hard blacktop.
The trouble I had with this is the use of 'shot' because that immediately brought a gun to my mind which was by far much more exciting than what the reality turned out to be: a punch in the face. Using metaphors and slang works better when you've already set the situation up so the reader knows what's going on and then they add to the reader's view of events. They shouldn't be used right at the start or you'll confuse your reader or give them a wrong image.
Also, if you're going to start with action, make it quick and snappy. Shorten your sentence structure and make it something that's going to really jerk the reader. And then start filling out details and working outwards so that they can see the whole picture. The other way to do it of course is to start out and work your way in. You can describe the scenery around and then focus on the two boys fighting. The choice is yours but make it a beginning that's going to be remembered.
Length
The real problem with this is that it's very long for what little amount of plot is actually give. The only impression the reader gets is that the persona is a kid who has been through lots of foster homes and has trouble follow him all the way. And I just covered everything in a sentence. So this isn't your main story but it's necessary detail, right? And it's detail you could convey in a shorter, more interesting manor. I'd suggest a prologue that centers on just the meeting with the foster parents and the social worker. I suggest that your main character isn't even in it. You say you created the fight so he could be moved on to his new home? Well the reader doesn't have to see it.
Show the foster parents at the end of their tether and putting up little resistance to the social worker's insistence that he be moved on. Show the social worker pretty much talking to himself, those make for interesting characters. The sort who doesn't perceive when people aren't arguing with him because he's so used to having them defend/ try to hold onto the child.
Of course I can't tell you what to do but that would be my suggestion ^^
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An article you might find useful: viewarticlebody.php?t=33840
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An excellent article that's not read often enough (No it's not mine this time XD): viewarticlebody.php?t=19719
I hope this helps a little,
Heather xx
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