Yes, you may definitely end a sentence with a preposition, when appropriate. There is no such rule against it anymore. I have worked in the professional world of publishing before I became interested in education, and so I am perhaps more aware of changes that have happened to the English language in the past few decades.  It is an obsolete rule that was based on Latin. However, people eventually figured out that trying to apply Latin grammar rules to English is just silly. 

For more information, please see: http://grammar.quickanddirtytips.com/ending-prepositions.aspx

and

http://lisakusko.wordpress.com/2011/10/10/lies-your-english-teacher-told-you/

While the technical answer is no, there are times when a sentence sounds better ending with a preposition.  Once when Winston Churchill was adonished by an aide that he ended too many sentences with prepositions, Churchill replied, "That is the kind of thing up with which we cannot put."  Churchill's obvious point is that ending the sentence with the preposition "with" sounds far better than the infelicitous sound of ending the sentence with "put."  The answer, then, is try to avoid ending sentences with prepositions unless it harms the sound and rhythm of the sentence.

No... technically. If the teacher allows it, it is much easier (and more contemporary) to do so. You are supposed to reword your sentence in order for that not to happen but what is the fun in that? I would TRY to restructure first

This sentence ends with a preposition. ;)

I would like to support and clarify previous correct answers: Yes, it's technically incorrect, but many people prefer to use the way that sounds best, which teachers often allow (or put up with.)

However, it is also worth noting that many prepositions become adverbs in a different context. Ending a sentence with an adverb is appropriate, and in some cases, that's why it "sounds good."

A previous example suggests we "register" at a hotel, but it's perfectly correct to say "I will check in." This sentence does not end with a preposition. In this context, "in" is actually an adverb that modifies the word "check." It's also an accepted/correct idiom. (More on correct idioms later. Hint: someone should ask about "SAT idioms.")

We regularly run into the situation when a sentence we initially write ends in what appears to be a preposition. Sometimes this occurs when we use phrasal verbs. These multiple-word verbs consist of a verb plus a preposition (called a particle in this usage) or adverb - which creates a meaning different from the original verb.

Often the phrasal verb terminates the sentence as in

“We will have the hotel keys once we check in.”

In this case, we could avoid using the “preposition” at the end by using the more formal word “register”, thus:

“We will have the hotel keys once we register.”

Read more at http://www.usingenglish.com, www.EnglishClub.com and others.

sometimes you have to

Technically, no. The previous answer is a common question, and not many people would instead say it differently. In most cases, the preposition isn't even necessary. E.g. Where are you going to? What'd you do that for? (Instead, "Why'd you do that?")

Where are you from?

I will advise you not to end a sentence with a preposition because at certain level you mentor will put a question mark in front of it. or will say what? where? who? However, some sentences occasionally, but not often are bound to be finished with a preposition.