Keep taking singing lessons, but understand that it takes time and hard work to learn how to use YOUR voice to its best advantage.
Meantime, I think I might be able to suggest a few songs, available as karaoke tracks, that you can try. First of all, I need to know your exact singing range. What you can sing COMFORTABLY from your lowest note to your highest note. Notes you can sing and sound good (on pitch) all the time, not just on "good days". It won't help to transpose a song UP if it becomes too high for you to sing. Plus if you are transposing a karaoke track more than a few steps, the track ends up being distorted. Next, does it have to be a POP song, or could you pick any other style?
I'll check back later tonight to see if you update your question. Remember that even if two singers have the same type voice, or even the same range, they don't necessarily have the same ability to use it.
Update: Sorry for the delay getting back with you. This is actually about the 4th time I've tried to enter an answer, then some weird popup thing on my phone knocks me off the screen. Then I get on my computer, and still manage to bump the wrong key and lose everything.
I should have asked to clarify if the teacher LITERALLY wants ONLY the ORIGINAL KEY or simply wants you to be able to sing on pitch even if you get a track that might be a different key or transposed or something? Does the teacher want the students to mimic the original artist? I'm not sure I understand WHY this music class exists or what it's about.
If you cannot change the key, is it possible for you to change the octave? If you ever noticed, in much sheet music, the vocal line is written on the treble clef even if it is meant for a man or a lower voice to sing it. It is generally understood that the low voice is supposed to sing the notes a full octave below where it's written, and females (higher voices, especially sopranos)are supposed to sing the music as written. So, if you were to sing John Lennon's "Imagine" to a karaoke track in the key of C (the original recorded key), you would be singing from an E4 to a G5. Likewise, other songs featuring high tenor vocal leads, such as those by Coldplay, might also work this way. For example, "Yellow" would have you singing from an F#4 to a G#5. If your COMFORTABLE vocal range extends into an E6, then this would be a piece of cake for you. You could sing in this range with absolute no strain, and keep things light, warm, and pretty. (But sing with feeling so pretty doesn't devolve into "syrupy").
No matter how things turn out, it doesn't mean you have the "wrong voice". You have the wrong teacher, or simply you may be in the wrong sort of music class. Without knowing what sort of class this is, or what the teacher has in mind, I can't really say. However, keep taking lessons, and perhaps after you get a good private vocal coach who can work with you on adapting your voice for different genres of music. By "adapting" , I mean you would learn how to sing in different styles using YOUR voice--NOT trying to CHANGE your voice, but change the way you use it and still keep it good sounding, and most importantly, HEALTHY. Not even all pop singers "BELT". Some might seem to have powerful voices only because they have good amplification behind them, so if you get to use a microphone when you sing, tell them to crank up the volume. Never push or strain your voice, and straining your voice includes trying to sing uncomfortably low.
Since much soprano repertoire--even classical or theatre music, may require you to be able to sing at least a B3 (the B below middle C) if not an A3, then hopefully with some training and some maturation (if you aren't 30 yet, there is a long way to go here), you might be able to pull off a semblance of some chest notes. Meanwhile, work with what you got. Remember there are plenty of girls (and boys) who are envious that you can sing high notes.
If you aren't restricted to just pop songs, you might consider a crossover type pop song like "Con Te Partiro" (Time to Say Goodbye). Excellent for light high sopranos. Only goes up to an A5, but people are bowled over when you hit and hold a note like that in a pure head voice. They think you are singing MUCH higher. Lowest note in it is a C4.
Here is a link for karaoke tracks. Not my favorite as far as quality (too synthesized--better companies like Sound Choice, Sweet Georgia Brown, and Chartbusters tend to use real instruments if there were real instruments in the original)--but large choice and affordable.
Other sites where you would buy and download music, such as i-Tunes or Amazon, will also have karaoke tracks for sale. Be sure to preview the track before you buy it to be sure you are getting the right version.
Speaking of which--did your teacher specifically say and mean "karaoke" or did he say a prerecorded track LIKE karaoke which can include any number of types of backing tracks that often even come in more than one key for the individual singer's voice? Makes a BIG difference.
Good luck. Hope I wasn't too late for your assignment, and at least gave you a direction where to look.