What do I do about swimming sports!!!!?

Tomorrow we have our swimming carnival and our teacher explained to us that we need to be there and swim our very hardest. But I am the worst swimmer in my year. I get tired really really easy and we have to swim a whole length of our public pool, which is mive!! I'm not sure how long it is, but it's very very long!! Anyway, It's tomorrow and I can do freestyle and backstoke but I am no good at toke, I can't do it fullstop. I want to do a width instead od a length of stoke, but i'm afriad my teacher will not let me, how can I convince him to let me. ALSO, I'm not to confident in deep water, in the shallow end I am fine, but we are starting of in the deep end, what is it like swimming in the deep end?? Is it any different, do you notice??? How do I keep going with my stoke if I get way way way to tired, i need some really really really good techniques that I can use to come a good placing and not get tired to quickly!!! Also, will I be okay in the deep end if I'm just swimming a normal stoke, or will it be different and scarier the the shallow end. Please any tips and stuff would save my tomorrow!!! THANKS!!!! OOXX