If your teacher gave your such an assignment she knows absolutely nothing about heraldry or genealogy.
There is no such thing as a family crest. A crest is part of a coat of arms. Coats of arms do not belong to surnames and except maybe in Poland they don't belong to families. In England,Wales,Scotland and Ireland they are granted by a specific heraldry authority to individuals not families. They are in inherited by individuals. In continental countries they frequently are assumed by individuals. Frequently more than one man with the same surname, not all necessarily related, were each granted their own coat of arms, all different. No one peddler who sells them on the internet, at shopping malls, in airports, in magazines etc will have all of them. They don't need to in order to sell to suckers. The only time they will have more than one associated wit the same surname is if more than one man with the same surname from different national origins were granted or assumed a coat of arms. Then they will have one of each and there might have been 50 others. Most men with that same surname are not entitled to a coat of arms at all as they do not have a direct male line ancestor that was ever granted or did they assume a coat of arms.
The surname product business is an outright scam. We have no laws regarding heraldry in the U.S. and a person is free to display any coat of arms they wish but to do so is actually identity theft.
Anytime you go into someones home and see one of those walnut plaques with a coat of arms on it on their den wall, over their fireplace or in their entrance hall it is okay to smile to yourself. However, chances are they really think it is theirs and it just isn't polite to laugh at people in their own homes about something like that.
Now you might have more than one in your family tree. That doesn't mean you are entitled tp any one of them. It just means that if you do family research and you find more than one ancestor that was granted or assumed coats of arms and if you have a book printed or even published, it is legitimate for you to put pictures of your ancestors' coats of arms in your book. It wouldn't be legitimate however for you to put in your book those that just happened to be granted to or assumed by someone with the same surname as your ancestors.
If you have any English lines that go back to early colonial days in the American South you have an excellent chances of having several in your family tree. In that case you probably have some distant cousins somewhere that have the ones the ancestors you share in common with them brought over from England 300-400 years ago. As a rule they don't display them. They aren't any good for buying groceries or gasoline.
Here are some links to heraldry authorities in several countries.
http://www.college-of-arms.gov.uk/Faq.htm
http://www.college-of-arms.gov.uk/About/12.htm
The 'College of Arms grants coats of arms for England and Wales
http://www.lyon-court.com/lordlyon/216.181.html
The Lord Lyon grants coats of arms for Scotland
Please feel free to put this on your teacher's desk and definitely give him/her the links I am furnishing you.