Simply speaking, no one but yourself should be in possession of your original diploma- or in fact any other important document.
Your documents can be translated by a “yeminli tercüman” (official translator)
After that, you must have them notarized by a notary. Notary offices are plenty and you can find them all over. You may then give your school these notarized copies.
Translation and notary services are often found in the same building or the same block. You can expect to spend around 350-400 TL (currently less than $50)
You must take your passport and the original document that you want processed with you.
Prudential First can help you determine which documents need to be translated and notarized; and direct you to the most convenient translators and notary offices.
Warning: Some schools will want to just see the original document. This is fine, as long as they don’t keep it. In recent years, some schools have been taking documents such as diplomas and passports and holding them hostage when employees express interest leaving. This is absolutely illegal and should be reported to the authorities.