Don't ask to anybody anything just prepare yourself first in class so everybody will come to you for his problem then see how he / she talking with you and give him responce in proper way.<br />
Just imagine they all sit there in their underwear, I've heard that helps...
start off by talking to smaller groups of people<br />
your friends , ahve them sit in front of you and give them your speal. then get more and more so eventually you have a large group infront of you.
Practice in front of your family
have some of your first crowds as friend or have part of them as friends (supportive ones) theyl laugh at ur jokes etc and ul step by step get better at it
I have the same issue, I do not do well in front of large crowds.
Operation Flash Point
It isnt easy but the trick is learn to just not give a ****