How should a guy behave around a girl that has rejected him? But due to circumastances (business, school, college, same classes etc.) the still have to see each other.

just act calm

Like his normal self minus a gf.

show no interest and stay away and dont talk

Comes with the territory, it should bounce off you
and be a sport about it. It happens to everybody.

Slash her tires.

Need to go after her in a subtle way. They usually reject you for some simple reason. But show her that you have other qualities that's not obvious . She might dislike a few little details about you, like looks or style, but you still have other qualities that she hasn't seen like toughness and resiliency, all that comes with personality. Maybe she'll grow into you with your persistence.

I guess if all that doesn't work, get yourself a bottle of liquor and go to a river and cry. She'll sympathize with you and probably offer you hot passionate sex.

How does one act around a girl who has rejected him??

Hmm, play hard to get??
Nah, that's not right

Hmm, cry and whimper while around her??
Nah, no way that could be good

I know now ... hold your emotions in check and behave as nothing has happened.
Shrug it off and move on to the next girl you're interested in.

As someone already said, act calm, under control. Never hate her, never even act like you have been rejected. Never let the friendship tie dies. Greet her subtle and quick, act like you always have stuffs to do. Act like you have lots going for in your life. If she ever needs help with something, help her quick like you still care for her, but not giving in or showing your down side. Look at everything positively.

behave normal like to a girl which u are not interested in the less interest u show her the more likely she will change her ming

Go find a girl who liks you just as much as he liks her easy.

Find a different girl and use her to make the first girl jealous.

Sitcom style

Let the hilarity ensue until it all blows up in your face.

This is how I've done it. If she rejects you, you immediately put her in the friendzone and you never let her out of it. You stick around and maybe she'll introduce you to one of her girlfriends. Never freak out on a girl that rejects you. If for some reason, comes back to you to take you up on your offer after a significant amount of time has passed, you make it clear that the opportunity is gone. A girl that comes back tells me that she's played field, feels like she's losing the game, and decided to cash in her chips with you. Hopefully, by this time, you would've done something with your life that increased your net worth.

Never ask a girl a second time to be your girlfriend. Don't take her if she changes her mind a year later. Follow these rules. In the end, you are safe because you still have your dignity, and you win.

jus ignore her for awhile until ur ok to be friends again

just act normal and cool about it like it ain't no biggie. Wipe it off your shoulders.