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 as a matter of fact in a lot of countries they don't belong to families.
There are numerous peddlers on the internet, at shopping malls, in airports, in magazines selling coats of arms like they belong to everyone with the same surname. Frequently more than one man with the same surname, not all necessarily related, were each granted or they assumed their own coat of arms, all different. No one of those peddlers 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 with the same surname is if more than one man from different national origins were granted or they 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 were never granted or did they assume a coat of arms and their descendants aren't entitled to one at all.
In the U.S. we have no laws regarding heraldry and a person is free to display any coat of arms they wish but to do so without documented proof that you are entitled to it is considered usurpation of another's identity. If you have pride in yourself you certainly don't want to take on another's identity.
The surname product business is a big scam. Not too long ago there was an ad running on TV for a company selling framed surname histories which in itself is rather shady as not everyone with the same surname will have the same family history. When surnames were assigned or taken during the last millennium it wasn't impossible for legitimate sons of the same man to wind up with a different surname and still each could have shared his surname with others with no known relationship. They were started for taxation purposes not to identify a man as a member of a family. Too many Toms or Horatios in the same town or village and they had to have a way of telling them apart. The man in the ad stated "a" coat of arms will be on it not "your" coat of arms. You see on TV the FCC can slap a company hard for fraudulent advertising. The FCC has no control over the internet or some merchant with a booth in your local shopping mall. The ad has stopped running and it is quite possible someone from The National Genealogical Society filed a complaint.
Here are links to various countries heraldry
http://www.college-of-arms.gov.uk/Faq.htm
http://www.college-of-arms.gov.uk/About/12.htm#a
The College of Arms grants coats of arms for England, Northern Ireland and Wales. They do not belong to families but to individual men.
http://www.lyon-court.com/lordlyon/216.181.html
The Lord Lyon of Scotland grants coats of arms for Scotland. There they are tough on a person displaying a coat of arms without proof that it belongs to them. A few years back there was a man with a coat of arms as a stained glass window and he was asked to remove it as it was not his. When he wouldn't officials from the Lord Lyon went into his home and removed it leaving a big drafty hole. Now, in Scotland they do have clan badges.
http://www.nli.ie/en/heraldry-introduction.aspx
http://www.heraldry.ws/info/article10.html
This is regarding Irish heraldry. In Ireland there are arms that are/were granted to individual men and there are clan arms.
http://www.regalis.com/onom.htm
This is regarding Italian heraldry
http://www.szlachta.org/heraldry.htm
This is regarding Polish heraldry. Poland is one country where they belong to families, dynastic families.
When you go into someone's home and you see one of those walnut plaques with a coat of arms on it just smile to yourself. The owners probably really believe it belongs to them. I will tell you what your parents would. It would be very rude to laugh or make some deprecating remark about that in their own home. You can buy key chains and coffee mugs at Stuckey's with someone's coat of arms on them.
Now if your teacher really indicated that you should have a coat of arms, please print of the links I have furnished you and give them to him/her.
Also feel free to print off what I have posted here and give to her. If you feel it would get you in trouble put it on his/her desk when he/she is out of the room.
You can design your own if you wish.