i have just been told i have to have a crb check at work?

[oooops]

i am a dental nurse i am currently employed my employer has never mentioned anything about any criminal records or convictions, i recently applied for another job and they told me that i would need a crb check before starting and asked if i had a crb at my current practice i said no and was then told that from april i would have to have one. when i was seventeen i got into a bit of a scuffle after a night out which resulted in me being charged and convicted of abh (assault) i went to the youth court and as you cann imagine im terrified that this will show on my crb check will it show as i was under 18 and im almost 27 now