Where can I find information about tax deductions for a person with a
dual career, specifically, a school teacher who is also a professional
musician?

Asusming that your teaching job is as an employee and the musician job
is self-employed, any information applicable to self employment in
general should be useful to you.  Teaching income should be reported
to you on a W-2, its reporting on the 1040 is the same as with any
other wages.  Self-employment income, generally, is reported on
Schedule C in manner similar to a profit and loss/income statement. 
The most important thing here is that you keep very good records.

I have a friend in your situation, a programmer by day and an opera
singer by night.  We deduct mileage, voice lessons, certain music
purchases and other related expenses.  We have not deducted home
studio space in order to avoid problems with the gain exclusion if and
when he sells his house.  An istrumentalist may be able to deduct some
instrument related expenses, but you will need to be careful that
anything deducted is not considered a personal, hobby-type, expense. 
The musical Schedule C should also show a profit, some years at least,
to avoid hobby classification.

Let me know if you have specific questions.

Carl Cochrane