Help!? How to increase vocal range and change the quality of voice? (for a male)?

Okay so im 13, i usually clified myself as a baritenor, so i talked to my chorus teacher and she said i was pretty much a b, since i sing lower than most boys at my school she said im able to hit a C2. Okay so i tried singing an A4 and i thought i sang it right, but then when i told my teacher about it and sang it to her, she said it was actually middle C, C4, and that's the highest note i can sing. Although when i sang it it felt higher than a C4 and sounded like it. She said i must hear my voice higher than it really is. So, when i sing up to a C4, it sounds really nasally and annoying and i want to change that to a more pleasant sound. I would be embarred to sing like that live, so i really do need to change the quality of my higher range. And id like to be able to sing an A4 (without falsetto) can any give me tips or help on this? please!