Clichés are bad. Clichés will severely harm your grades if you should use one. Now, I should probably make it really clear what I mean by a cliché, so I have a few examples:
1 - She was quiet as a mouse.
2 - They avoided him like the plague.
3 - Her skin was as white as snow.
It seems that most clichés are also similes, but that's not the point. I only partially agree that clichés have no place in fiction, I think an important distinction needs to be made here. Clichés in the narrative are not okay, it's lazy and sloppy and almost cheating on the part of the writer. Clichés within dialogue, however, are totally fine.
Dialogue is supposed to represent the way that people speak. It's not always an exact replica, obviously, because that would be boring, but it has to be close enough that it's realistic. People use clichés when they speak all the time. If we were talking and I said "She was as quiet as Winchester Cathedral on a Tuesday afternoon in February," you'd probably think that was odd. If I said "She was as quiet as a mouse," you wouldn't bat an eyelid. So why should there be a different rule for characters?
Sure, if your character is well-spoken and makes a point of having a wide volcabulary and an unusual way of speaking, then don't use clichés. If your character is, you know, normal, there's no need to be afraid of the odd cliché. Just remember, everything in moderation.