When I talk to girls in my class, they feel like im just this friendly guy, how do i put that flirty feeling into the way i speak to them

read alot... study the art of Innuendo ...be funny be clever n never be sorry for being who u ar

Have you tried flirting? Below are some examples, work little stuff like this into normal conversations.

Cause a scene (just between you)
Tease her about something she wore that day
Point to another girl and ask her if she would hit that!
Tell her she is number 4 on your list of cute chicks in the class.
When she smiles, flips her hair, makes a face, or whatever, tell her "Do that again", when she says do what, just say nevermind. Change the subject. After a little begging, tell her to flip her hair again. The smile when she does it and asks why!
Challenge her to a contest where you spin in a circle one time. You do it first and ask her to score your performance. Then when she does it and asks her score, say I dunno, I was totally checking out your ass!
Ignore her a bit...
Be sarcastic!
Ask her if she needs a hug when she acts upset with you.
Ask her if she knows any cool chicks!
Tell her you need her to be your wingman this weekend.
Touch them when talking to them.
Do things that require touching hands
Ask to see how much taller you are than her, stand back to back. Afterwards, tease her about rubbing her butt all up on yours.

Just be a guy who is fun to be around, but always throw a little "sexual" hint into your interactions!

Peace...

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Crypto...
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All girl's are Freaks...It is your job to bring it out in them! - Crypto

You need to stop bending over and letting her ass fuck your brain! - Heywood

Yep as Crypto says, a lot of it is not about changing your personality or how you act etc. it's just about "sexing it up" a little bit. This depends on the situation, location, type of girl you're talking to etc. Sometimes you can just go full throttle and chuck a lot of sexual innuendo in there strongly. Other times it can be something as simple as changing your own mindset.

If you're naturally a friendly guy who, I assume, can talk well and hold a conversation without feeling shy and going quiet, then you've got the hard part sorted already. Just think in your head that you're some sort of smooth sexual bloke. Focus on that, not on what you're saying. And then even if you're saying stuff you normally would, there is a different vibe there, in your body language, how you say things etc.

It's really difficult to isolate girls in school if their in a set, so how would I execute opening the set and then making them I realise I want to talk to a specific girl?