How can i sing higher?

im male 15 and im in the choir. since im older than most of them, my voice is incredibly lower compared to them, and recently i found out that i can't sing as high as them!!! the choir teacher is thinking of making me quit because ican't sing high enough for the songs. my voicve is seriously deep and when i try to sing higher notes i just can't and i the phlegm in my throat is like vibrating.... what should i do to fix this problem??? like what should i drink/eat.