I remember having this problem at school- it seems we do perspire more as teenagers, and keeping clean is far more of an issue than it was as children. Make sure you wash under your arms really well every morning, and lunchtime if you are worried. And you could use an anti-perspirant rather than a deoderant, so it will actually stop you from sweating. Make sure you wear cotton clothes (not man-made fibres like nylon and acrylic) and change your shirt every day.