Turn by itself means to change directions, as in "turn from right to left". In this sentence you should say "turn out to be" or "turn into".

It's not really correct or natural- a bit confusing. I assume the meaning is that they are not yet teachers, but when they become teachers, they'll be good ones. In that case, it's better to say, "They should turn out to be good teachers."

They will become good teachers.

As you work on your English grammar and usage, please don't ignore the importance of punctuation in your typing -- commas, periods, question marks, quotation marks, etc. They MUST be spaced correctly. (You did not use ANY spacing in typing your question.) Look carefully at the answers you receive here and notice where the spaces are placed. Most are correct and you can learn from them, but not always! For example, in the first answer below, everything is correct except the hyphen after 'natural' in the first line. She should have typed a dash, like this -- not a hyphen ( - ).