Tell me why song and other songs for the recital

In the song tell me why,I don't want to sing the whole song for my recital because I had prepared a list of six songs and gave mock performances of these songs at home,and as I practiced these songs,The total was a 25 minute performance maximum which I will be doing for my final recital in two weeks time.When I had downloaded my songs to MP3, I didn't know how to cut the parts of the songs tell me why,but before or when I start performing on my recital day,I will have to tell my teacher to stop playing the song and move on to the next song.I am only singing most of the song and how I am going to sing is I am going to do the whole F sharp major scale of the song and towards the ending of the song,I am only going to sing one paragraph when the scale of the song changes from F sharp to B, and I am done with that song and continue to go ahead to sing the other songs for the rest of my performance for my recital.