Can anyone explain to me how to be able to be able to do whatever with a girl and not get into a serious relationship? (I'm sophomore in high school.)

To be honest I was in a different mindset in high school. I had a highschool sweethart at a time and was set with it.
Right now I'm very honest and upfront with a girl about it. Just make sure that she understands right away that you're not looking for a commited relationship and but that you like her and that you like spending time with her. Not all the girls will go for it but when you find the one that is down with that its fucking awesome.
Try turning a casual dating relationship into a FB one, or a one night stand into it. It's the easiest way to go. And to make sure it stays a long term FB relationship, just make sure you don't start showing relatinship behavior. Such as spending too much time with her, on the phone or in person, meeting each others friends etc.
It's not easy but don't give in.

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This game is automatic. You give it to 'em one time, they come back like addicts.

From what I've heard, FBs aren't very common in highschool, mostly because girls at that age have a different mindset and typically aren't open to that sort of relationship until they are around 20+.

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"The 'Brick Walls' are there to allow you to prove how badly you want something!" ~ Randy Pausch

~ Rye

There are plenty of high school girls with that mindset. Most of them are FB's with older guys they can't have their mommy/daddy know about.
Typically if you can have sex with a girl without going on a date your in charge of its initial seriousness. They might not go for it so it could just be a one night stand, but they might. I think the thing to do is find out there reputation first to know who goes for it and who doesn't.
I've never enjoyed the idea of friends with benefits, so thats about all the advice I can give.

to make this a bit less complicated,
simply just be her close frds,
and when u get the chance to be alone just w/ the 2 of you,
touch her, arouse her or wut not.
and if get it on, do her.
but never say a word about commitment~
but that'd be a real bastard huh~
considering u want frds with benefits, it's not a gd start~