I hav to sing in front of my class nd im really nervous whut should i do ?

the reason is cause my friends hav been sayin that i cant sing butt i really can the reason they say that is cause when im around them singin i just hum nd dont really sing but tomorrow i hav to lead my class in a song and my teacheer told me i hav to stand in the front of the class so i cant hum i hav to really sing nd my teacher said she is gunna b watchin me to make sure that im really singing.